Well, no bullshit here - I would be freaked out too if I were you. I really can understand that this is the last thing you need in your life. But you will be safe, right? Your Grandpa will be there and so will your T. I'm sure they will do everything possible to support you, and remember that the DA's office isn't going to judge or blame you. They just want details to help them in their investigation.
Do ask for a bit of time alone with your T before the interview. Keep a list of your questions as you think of them this weekend, and ask her about these things before you talk to anybody. If the people talking to you have any business at all talking to abused teenagers they will understand how important it is that you feel as safe as possible before any discussion begins.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)