Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus.    

----------------------------------------

If Sans had the capability, he probably would have gone right over to that robot and smacked him clean across the face right now, right into next week. Because no way in fucking hell had he wanted to tell Undyne, and now he was just glaring towards the ground, as the fish fell silent at such a thing being mentioned. It was going to happen, wasn't it? Whether he liked it or not, he was still going to be put in prison, and he was going to have his child taken from him. He would lose everything. 

"why did ya have to fuckin' tell her...?" Sans growled towards the floor, not wanting to look up at anyone's faces. Probably because he was in the midst of having his own little freak out because now he was imagining the possibility of being separated from his child. Before meeting Frisk, he never once considered having a family. Even when he found out what had been going on with him when he felt unwell, he felt fucking terrified, and he felt like he wouldn't be able to do it. Him, raising a child? It felt like an impossibility. It still did a tiny bit. But then he remembered, he wasn't alone, he had Frisk, and he wouldn't be raising this kid all by himself with no one to help him. They would be a family. "she's just gonna force me behind bars for life... and then she's gonna take them away from me..." Sans muttered, wrapping his arms around himself as he actually found himself getting teary eyed. Undyne blinked. 

"I didn't--" Undyne started, only for Sans to look right at her, looking like he was ready to burn a hole right through the center of her forehead with just his stare alone. Those tears that started to sting at his eyes slipped down his cheekbones, and had he taken notice to it, he might have saw some kind of look flicker across Undyne's eye. He growled. 

"shut the fuck up. all i am is just a stupid target to ya. just a bad fuckin' person, who hurts people. yeah, maybe i have killed people... but guess what? i don't give a fuckin' shit. because they were people who clearly deserved it, who were wastes of oxygen; people this world would be much better off without..." Sans seethed, ranting on as he just needed to get all his rage induced thoughts out before he would end up putting his fist through a wall. Frisk listened to all that he was saying, as she slowly approached him, wanting to be at his side to calm him down. "you're just gonna take 'em from me..." He repeated. 

Frisk stood, actually watching Sans slowly back away, moving towards the back of the cell where he ended up turning his back on everyone, as he was beginning to sulk. She couldn't blame him. He was emotional, and just the thought of their future being torn from them made her want to be emotional too. Exhaling, her eyes trailed to the quieter fish, and Frisk decided to step closer to the bars of their cell. "Please, Undyne." Frisk pleaded, looking Undyne right in the eye, with her own desperate blue ones. "Sans: he's not a bad person. No one here is a bad person. They've done questionable things, I've done questionable things... But we aren't terrible people." Frisk muttered.

"Frisk... You've killed people." Undyne answered back, in a more glum tone. "I don't enjoy this, I don't... But..." 

"Bad people. Yes, killing is still killing, but they were bad people." Frisk replied. Sometimes you just had to do what you had to do to survive, or to make a living, or just to protect the people that you loved. Frisk did what she had to do that one day where she had shot that man to protect Sans, and had she not done that, he likely would have been killed. When Blair had captured her and Sans, they had to shoot their way out. He had killed a few more people, but only people who knew what they were getting themselves into.

She wanted to believe in good morals like she always did, she really did. She wanted to play the whole "killing is wrong" card, as she tended to do when being thrown into horrible situations. But if there was one thing this world had taught her, sometimes you just had no choice. It wasn't about killing innocent people, that was a whole different picture, a picture she definitely did not agree with. Quite frankly, she could stop caring for the terrible people of this world that would gladly feast on the suffering of the weak. Just the way those thieves broke into her house all that time ago, and murdered her parents right before her eyes. They took away the lives of two people who never even deserved it. 

"These people, they saved my life. If it wasn't for them, I would be dead, starving to death on some street corner somewhere. Gaster - he might have been gone. Andrew, Blair, being brought into this group took them away from worse things." Frisk told her seriously. Blair's fate could have been a whole lot worse had she continued to stay with that group. It was already fucking awful enough what she had had to go through in the past; what would have happened if it was just decided she wasn't needed anymore? No one had come to rescue her after all when she was captured, so it just showed where they stood. And Andrew, he had a family that just wasn't good enough for him, that probably didn't really make him feel that important. Why else would he have pretended to be his twin brother; he wasn't good enough to his family.

"Just please... Please don't take Sans and my baby away from me. Please, please let us make a deal. I can't lose everything after feeling like I'm finally getting what I've been missing ever since my parents died." Frisk said, as tears began to stream down her face. "Please." She pleaded. Undyne stared silently back at her, at least until another voice was there to interrupt that silence.

"U-Undyne? C-C-Can I talk to you, a-alone for a, um, s-second?" The stuttering dinosaur spoke up, after having shortly reentered the room; she had been watching and listening to what was happening. Undyne took a moment, saying she would be right back, right as she went to leave the room with Alphys. The yellow dinosaur suddenly became nervous as she fiddled with her hands. "I k-know it's not really my p-p-place to say... But I think you s-should listen to them... T-They don't seem l-l-like terrible people."

"...Admittedly, it is strange, actually seeing them getting so upset. I do feel guilty, knowing I caused that..." Undyne murmured, looking elsewhere with that one eye of hers in thought. When you were going after someone, and you didn't really know anything about them, and you just saw them as a terrible criminal who you were supposed to arrest, it was easier to alienate them. To do what it took to capture them and make sure they didn't harm anyone. "I didn't know he would be carrying a kid in his soul though."

"...I-I know you have b-been pretty passionate a-a-about this case... A-And it took you a long time t-to catch him. But now t-t-that he's finally here, I c-can tell you d-don't seem happy with it." Alphys said towards her wife sadly. She also personally felt some kind of guilt that they were being locked up, and they were getting pretty upset over the possibility of having their child stolen from them. She knew it was their job to stop bad people, but these people just didn't seem... bad. 

Just by Sans yelling at her and breaking down, Undyne could see how much it meant to him to look after that child. The child that she could end up stealing away from him if he went to prison. She had arrested many people and unfortunately torn up families because she really had no choice, as it was to protect innocents. When wanting to go after Sans, she just thought he would be this heartless, careless monster who got a joy out of murdering people. Because she had arrested many people like that. But this guy, seeing that look in his eye, it was difficult not to sympathise with him for the situation she herself put him in.

Her job was important to her, it was extremely important. She had done so much to get to this point, to be where she was. But with what she was feeling, and the look Alphys was giving her right now, was it really worth it? She had already possibly driven Mettaton and Papyrus away, she couldn't stand to upset Alphys also. The fish silently returned to the cell room, catching the attention of the prisoners the moment she entered with the dinosaur following behind. She narrowed her eye, staring at all of them, until she finally spoke.

"Let's make a deal." 

She was giving in.