D.W.
D.,
I think that being pregnant can make one a bit crazy! At least I can attest to that, as well as post-partum. I wish I had realized a bit not to take myself so seriously, and to take less stock in my volatile and intense feelings.
My thinking is that your fiance and his mother are your family. Whether or not you get married, they will always be a part of your kids life because this is their child and grandchild. Having someone overly helpful and wanting to buy stuff and feeling ownership really is sweet. What you need to remember is that this is your soon to be husband and your baby.
Most children growing up would be jealous to have so many interested parties clamoring to caretake and love them and I'm sure that this will only prove to benefit your child and give him/her the sense of love and stability every kid needs.
I've always thought that unless someone is hurtful, cruel or evil one can deal with a lot of purely "annoying" behavior. Your soon to be mother-in-law did a pretty good job raising your fiance, I'm guessing, and I bet she just wants to be a part of your beginning family.
Remember, hormones are powerful. That your folks feel the same way probably is reflective or their excitement as well as desire to spend as much time with their grandchild as possible.
Hope that helps,
D.