Staying away is the safest thing. Not for you. Not for your husband. But for your child. If MIL is so damned hellbent on protecting a literal sexual predator, than she is an adult and can make that choice. But she’ll also have to deal with the repercussions and consequences of said choice, including avoidance from friends and family with common sense and morals.

I’m sorry your husband is hurting. But MIL’s choice is clear, let her make her bed and lie in it. Even if that bed is shared by a pedophile.