She is in the wrong. She is being weird. Huge red flags. Your baby. Your choice. Good to hear your OH is on your side. If she keeps going on and on about it and you're still really uncomfortable, I would tell her so. Something along these lines.......
MIL, I appreciate that you are excited about the baby but as I have said before, I will be exclusively breastfeeding. This is something I'm not changing my mind about. If you continue to ignore my choices and make me feel uncomfortable about said choices, I'll find it very difficult to spend time around you when the baby is here, as I know you are not respecting the boundaries DH and i have set. There will be plenty of time for you to spend cuddling, playing etc with baby. And when baby starts weaning, you can be involved then.
And when baby arrives, and she STILL keeps saying it, call her out.
MIL, I have spoken to you many times about this, and you have ignored my boundaries of what will happen with the baby and feeding. This means that because you can't respect my boundaries, you won't be invited over again. I need to be comfortable in my own home with my newborn. Goodbye!
I hope this all works out for you. But from experience, please stick with your boundaries. And make sure you and DH on always sticking together with your choices. Do not falter (give an inch, take a mile cones to mind). And call them 'boundaries'. And call it 'disrespecting the boundaries'. If you call them something like rules, rules could be broken. Boundaries do not move unless the person setting them says so.
Sorry for the long reply, but I have been there, done that and got the t shirt. And I didn't stick up for my choices. I seriously regret it.