over involved family?

Keri • "Happiness can be found even in the darkest of times ; if one only remembers to turn on the light"
I appreciate every little bit of help I get from my family. But sometimes they are overstepping their bounds. I feel like i have no say in my own child's life. And when I politely say for them to back off a little bit because I'm becoming overwhelmed and stressed, they get very defensive. 
Any tips?