I think you should hold off.
#1) Realistically, she can't pay for her own plan because you can't even get a workers permit until 16 or so in CA right?
#2) My sister got one around 14 because the School bus was pretty unreliable so my mom gave it to her to to call in case the bus was late. I dunno if that applies to your sister. But I don't think not getting anything for her junior high grad is a good justification.
I think you're doing too much, with your last 2 posts I've read, it seems like you have a lot on your plate in terms of financial obligations. Adding another cell phone bill is another long term commitment and you never know when they're going to go over their minutes (I myself have been guilty of $120+ in high school). Everyone needs to learn the responsibility of doing chores. Sure good behavior should be rewarded, but Im sure you can do it without a cell phone. Still its your money, so in answer to when is it too much? When its out of your means. If you can do all this, and still have some saved for a rainy day, then it should be alright.
EDIT: damn, missed the part where you decided already. oh well.