My parents did not want me to join a sorority, but they did not have a say in the matter because I paid for everything myself. (They did come around after awhile when they saw that nothing horrible happened to me.
) So, I would say that the only way to avoid them exerting their influence over you is to save up your money and pay your dues yourself: your money, your decision.
I do know at least one woman who was pushed into joining her mother's sorority. She was very good friends with my sisters and had expressed a lot of interest in joining AGD. However, her mom decided to visit during recruitment and offered to help her sorority, and sent her daughter letters on her sorority's notecards practically every day of recruitment. (Basically, her mom dirty rushed her.) The girl ended up in her mom's sorority and was ok with it, but never really put much effort into it and, in my opinion, never really got the full experience that she could have if she went somewhere she wanted to.
Bottom line: you'll know what's best for you, and you need to be able to stand up to your parents and make this decision on your own.