2. 「お姉さんの一番好きなところは何ですか?」or「妹さんの一番好きなところは何ですか?」, depending on whether the sister is older or younger than the person you are talking to.
1. I guess「お姉さん(妹さん)との一番の仲良しエピソードは何ですか?」or something alike is what you are trying to say, but I'm not absolutely sure what you actually mean by the first English sentence.