MIL Issues

🍂Ash🍁 • My husband and I have 3 kids between us and just found out today (3/14/16) that after 2 yrs TTC we are pregnant!!
Let me start off by saying I'm 34 weeks pregnant. This is the first baby between my husband and I. He has a 9 year old daughter that we are currently seeking 50/50 custody of. I have 2 sons; 12 and 8. 
Every since we announced we are pregnant my mother in law has been too much. She started off by buying clothes, diapers, etc. was was nice but then she started keeping them for her house. Let me add, I'm going to be a SAHM so there's no need for ever having a sitter. MIL doesn't work and is on a limited income but continues to buy baby stuff..only for her house. In the past couple of months she's bought a swing, crib, changing table, bouncy seat; she literally has a nursery set up in her house. The last straw was when I noticed on a FB buy sell trade page that she was getting a used car seat. I told my husband. He called her and blew up over this. We've told her sooooo many times to stop buying baby stuff for her house. She informed him that she NEEDS this car seat because she WILL be taking him several
times a week whether we like it or not because no one is keeping "her" baby from her!
I'm seriously losing my shit with this woman. What do I do?!? We've had several talks with her and she's just not getting it.