1 你有多少兄弟姐妹？ 2. 你有几个兄弟姐妹？ Both of them are correct,the same meaning. But for"她妹妹怎么样？"，normally,we use it when we know her sisiter is sick,means"What about her sister?", or someone never meet her sister,but they want to know her sister's appearance ,character and so on,just like"What's her sister like?" If you want to say"How is her sister?", you'd better use"她妹妹好吗？"

all correct～😀

Both work，perfect！！！