As a parent, your FIRST and ONLY responsibility is to your child. 

If your mother wasn't willing to step up for you as a youngster, do you expect her to suddenly change her ways now???

Be honest and straight forward with them about your concerns. If they fail to see the truth, then NEVER let either of them be alone with your child.

And even if they agree with what you've said, that could be a hollow statement just to get what they want.

And regardless of their response, NO SLEEPOVERS......EVER!!!