There is alot of ways it could go. All 3 of you could make it into the vault and then you awake alone and your family is gone. You have to find them. Someone has taken them, the insitute. They pit you against the Brotherhood, if you want, or you can use the Brotherhood to go at the institute to get them back. There are so many possibilities. I am curious if they will actually kill them off in the beginning, seeing as you can customize both characters before continuing on with the one you want, i am gonna say they will survive.
Don't know if you remember my earlier post where I said I'd hold off on what I think the whole "sole survivor" thing means for the main story, but let's just say, hmmmmm...very insightful of you.
BTW, I'm an old guy with wife, kids, dogs, guns, mortgage, mediocre golf game, blah, ...oh and happens to be a veteran. The main PC is pretty much my doppelganger...so no real issues with that here; but I do see why others may not dig it so much. Being on my own without the weight of the world on my shoulders, is just too far in my past for me to relate to.
But, I have no problem seeing it from other's perspective. I for one, do prefer a blank slate like Courier 6.
EDIT: which brings me to my real point: I'm wondering if you can make a baseball bat with nails AND those saw blades.