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.
Oh wow your birds are stunning! Baby number 1 looks just like my Marli when she was young, but she is a yf2 so her yellow spread through her purple and muddied it as she molted her baby feathers, she is a pastel colour, either greywing or dilute I'm not sure which. What beautiful chicks! And the parents are gorgeous! I have never seen a budgie like the mother, awesome
Edit: I have tried editing to add a photo but can't, so I've put it in the gallery and hope the link works, it's a picture of Marli (top) and Marlow (bottom, the pied) in their adult plumage after Marli's yellow spread during her first adult molt, she was a beautiful bright lilac colour as a baby, and in person you can still see the violet factor clearly but most of it is muddied, looking at the photo I think she is a grey wing. I searched the net for a picture of a similar budgie, has amazed me to see your chick and how similar it looked to Marli as a chick thank you so much for sharing