Sibling looks like a YF 2 opaline sky blue. Can you post photos of the parents?
It seems to me baby 1 has 1 dark factor and baby 2 has no dark factor. Violet factor is hard to see. If only one parent has violet factor in the single form, it will be hard to see unless on a cobalt bird. But with the cobalt baby also being greywing, violet will be hard to see. It's also difficult to see SF violet in sky blue birds. If you can post photos of the parents and maybe some of the chicks from the front where we can see the bellies, we could guess better.