Jealousy because she takes your attention and they don't get as much and sometimes insecurity because what's wrong with your family that you have her instead to confide in. Do nothing these feelings they have are not your responsibility, so ignore their feelings, but don't ignore them, treat them well too.
Probably because it's a friend of your dads, your mom and family probably think there is something going on between her and your dad. Your mom is also probably feeling sad that you and her are not close like that. your mom probably needs reassurance that you love her and this woman is not a replacement for her. it would hurt to see your daughter that close with someone else.