1) In fact, with TMG8's mass add screen, I don't need to go the "add sibling" route.
2) Hmm, seems like assuming the second parent when adding a sibling IS making assumptions about my data. I may be adding a half-sibling, or I may not know which of a man's multiple wives is the mother. That latter is a situation I'm currently staring at, in fact: I have a man with two wives, a dozen or more kids, no marriage dates and absolutely no clue which wife is who's mother.
In fact, if taken to the extreme, absent certified paternity tests or witnesses to the actual consumating act, every time I add a child to a marriage I'm assuming that the wife wasn't busy doing early-morning extra-curriculars with the milkman. There comes a point where scepticism becomes unreasonable.
In any case, a design modification alongs the lines I proposed above makes no further assumptions about the data than the current design; it simply saves an extra click.