I don't want to drive my boyfriend's family around all the time.
Why don't you want to drive them around all the time?
They're taking advantage of me and I'm tired of it.
I could understand them asking you every once in a while.  However, it seems like they're doing it a ton and your SO should be preventing his family from taking advantage of you like that.  It's not your responsibility to drive them around all the time.
I'm telling them that I'm not driving them anymore, it's time for them to understand they're taking advantage of me.
Talk to your boyfriend and explain the demands on your time and the pressure his family is putting on you. Ask him to talk to them and set up some boundaries so that you don't feel like you're being taken advantage of.