I would like to add that there is no way on earth i would leave my baby with my MIL over night. Maybe when he's about 2 or 3 years old, not before!
I leave him with my mum for up to an hour, usually just in another room whilst i do some chores and get some space. If she wanted to take him out for a walk for an hour I would be ok with that, but only because he knows her now and she plays with him all the time. My MIL only visits occasionally and i wouldnt feel comfortable that she knows how to calm him without me there.