Auto tune is almost ubiquitous these days but please read the second part of my comment, the part where I said "it wouldn't surprise me if no auto-tune was used here".
There's still the nurture side there. She's going to have music around all the time, high quality, decent teachers.... You'd probably have to take her out of the that environment and see what happens. Or take someone else and stick them in that environment. If you had a group of clones, two sets... One genetically related, one not. Or maybe more clones with variations of non-related. Leave some in the original environment. Take some out to another music/arts environment. Stick some in a non-music environment. Then compare. Probably no surprises though. Better environment = better results. For genetics, he's already risen to the top along with other family members. If they weren't good at music, he wouldn't be in it still. Someone less genetically geared up for music didn't "survive." I'd still think it's both nature and nurture.I wonder what kind of pressure there would be in an environment like that. Even if no one demands anything, I would still think there's something implied. It would be like a technical/appearances type of pressure. "You know you were off on that note, right? Don't you hear that?" If something had been technically off for a while. Some kind of love of music would be kind of required too I would think. If everyone else in the family loves music... then everyone in the family would, right?