Unlimited talk text and data for me and it's 75$ a month. That's one phone and could be cheaper. 100 for a phone on a 3 person plan means either you are paying 3/4 of the bill or your parents have an absolutely asinine phone plan that you would be better off leaving anyway.

Do yourself a favor and sever all financial ties with your parents and continue your relationship as equals. Your mom may think of you as her little girl still but your not. You should be on your own and I don't just say that for your benefit. The longer your mom thinks it's acceptable to act this way the harder it will be to curtail the behavior. Change nothing about the way you are around them except the finances and see what happens. If you continue normally then there is no issue. If your mom makes an issue out of the money then you can dig into why this financial control is so important to them.