Regarding #1, that is pretty natural in general when kids move from elementary to different middle schools. As the year progresses, she will become more focused on her new friends as well.
Regarding #3, it really isn't their business. You can simply say they are both adopted and are not biological sisters. It is also not unheard of for biological siblings to be less than 12 months apart - especially if that is the case for your two DDs. Even if they are a bit more than 12 months apart, it sometimes happens that they land in the same grade anyhow.
