Board Thread:Speculation House/@comment-4830106-20140806180925/@comment-24198837-20140817220759

My guess is that Winter is a full sister, but the age difference between Weiss and Winter is extreme. Either she was too old and already moved out / had her own life when Weiss was growing up, and couldn't protect her from whatever (apparently) horrible stuff is going on in the castle, or she was born when Weiss was already older and still isn't old enough to offer that companionship or console Weiss needs.

However, an identical twin - one who's opposite to Weiss in every way (Winter wants to run SDC while Weiss wants to be a huntress, Winter is meek and shy, whereas Weiss is not afraid to speak her mind, etc.) - is an interesting proposal, ESPECIALLY if Winter was daddy's little angel, and suffered none of the apparent abuse Weiss had to go through. That would make Weiss all the more lonely - she has an identical twin, but she'd be better off talking to a brick wall than confiding in her.