Okay, this is pretty simple. Sky and Peril were in the same egg, so Sky must be immune to fire. But I also have a slight idea how they might find out they're siblings. I haven't read book 15 yet, and I think/ hope this will happen. So, I reckon one of the scavengers throw some kind of burning stick at him, and the fire doesn't affect him. Qibli is part of the super-stealth team. He already knows Peril's backstory, and when Wren explains how she got Sky, the dots connect in his head. Similar backstories, fire immunity for Sky and firescales for Peril, and how they even look slightly alike (weirdly blue eyes and scales a strange shade of orange). Yea, I know it won't happen like that, but it should!

Lol, I need to have a better sleep schedule than just writing at three in the morning