I put my sisters sim card in my Blackberry. Got message that said it's been activated. ATT. Should be worried?

Yes, it is absolutely possible that her data plan could be changed. Any time a SIM card is put in to another device and the device is then powered on the network captures the IMEI of that device. If her old device will power on then she needs to put the SIM back in it and turn it on. If it does not then I would suggest she call Customer Service at 1-800-331-0500 and explain what happened