Okay, I know in most KFC(Kris-Frisk-Chara) AUs that Kris is the oldest sibling BUT CONSIDER
Kris as the youngest sibling
Chara as the oldest(not counting Asriel) and Frisk as the second-oldest
Baby Kris
BABY
KRIS
B A B Y
K R I S
B

A

B

Y

K

R

I

S

Of course that does mean that Kris would have to be found and if you all get far enough along then you can see the accident that made them mute if you don't already know