ok i have at&t with my mom and i want to get a t-mobile sidekick and i was wonddering if my mom still keeps her at&t account for her an my brother can she openm another account with t-moibie? idk bacaeuse how would t-mobile know of you have a service with at&t, anybody with past expierences or anyhng or just know let me know thank you.

That isn't a problem. If you want, you can have multiple acounts at different providers. They don't care, all they care about is that your bill is getting paid. They may question it when she signs up for her new account, but they won't stop her from creating an new account,and they can't make her drop her AT&T account.