Verbal agreement with family

My sister needed a cell phone so I put her on my plan. Well she lost the phone. So I called the service provider to suspend service until we either located the phone or replaced it; either way I didn't want a phone, technically under my name, out there being used and costing me money. She agreed to pay for monthly service to me. Well instead of replacing the phone on my service she went onto a friends cell phone service and now refuses to pay the cancelation fee. What rights do I have here? She agreed to pay verbally but nothing in writing. Don't think I am a horrible person for trying to take my own sister to court but believe me she does this stuff ALL the time. I just didn't think she would do it to her family. Any advice you have would be great.

Thanks

You two were obviously raised in separate families or you would have known she would be inclined to do this and wouldn't have agreed to do so in the first place. Do you really want to have hard feelings over a few hundred dollars for the next 50 years?
Do a creative solution and have her do a couple of favors for you and call it good.
You will both be much better for it. Yes, forget the money, save the friendship.