C.G.
Ugh! Passive aggressive is so hard to deal with. I'm sorry.
I totally understand what you mean about her not knowing boundaries. She is keeping these clothes and things because she is ignoring what you want for your baby.
Here's a thought: Is there someone else in the family that could interpret for you two if hubby and his dad don't want to?
I would not let her babysit. Do a family get together, meet at a park, go out to dinner together. She can see the baby but don't leave her with MIL until you are convinced she will abide by your wishes for how to raise your child. Hiding shoes is just the tip of the iceberg if you let this go on.