My Mother in Law is being possesive over my un born Baby.?

I'ved been married to my husband for almost 3 months. I am 6 months pregnant. ever since we told his Step Mother that I am pregnant she has been calling it "her baby". she raised my husband since he was 4 and she has never been able to have any children of her own. She has been trying to take over my experience of being pregnant and having a baby. She doesnt understand what I go through with my pregnancy etc. And his step mom and dad are well off...and she has already started buying baby stuff. At first I thought she was doing what a normal grandmother would do, buy us things for our( my husbands and mine's) baby. But she is keeping it at her house. She bought a brand new baby crib, expensive bedding, expensive baby clothes spoons, bibs, pacifiers etc. She told me I have to pump her enough milk to feed "her" baby while its at her house. She wants to be there when I give birth and I dont want her to be. I dont mind her visiting with my baby, but I am the mother. What should I Do?