Can we change the appearance of sibling and father?

Supposedly, we can modify father/sibling to an extent. Of course, I find default Sara rather good looking, Alec looking like a badass. With Scott, uh... I won't judge that. Haircut's off, IMHO.

They said to a limited extent, which probably means they'll reflect your skin tone.

I remember reading somewhere (I don't remember where) that you can fully customize your sibling, or alternatively, if you don't feel like doing that, you can let the system figure out his or her appearance based on your character's appearance (the second option sounds a bit like how PC's son's appearance is determined in FO4). I'm not sure to what extent we can customize our father.

Does anyone know if we can just tweak them a bit , Like different colour hair or eyes ? Or is the default set ?

Hard to tell if the default characters will be customizable: I kept thinking they were, but I dug up the original tweet and it's here:
Almost sounds like they may not be? Kinda weird seeing how all the presets will be based on scans, not just the defaults, so why can't the defaults be customized too? Maybe they've clarified the point since then... I do know we spotted an NPC with Sara's ponytail though, which suggests the hairs at least will be available in the CC for more than just the default.

I think you can fully customize your sibling's appearance, but dad's appearance will be generated automatically based on how you make the siblings look. Related tweets:
And here's more:

Haha I think the "to an extent" means he won't look like your twin. Anyone hear every play the Sims and create a child by mixing the mother and father's DNA? It'll be something like that I bet! Also they said we could have an Asian Ryder, so the dad will change to match that too I'd assume. Sort of like Hawke's family.