Soon to be Monster-in-law

My soon to be mother-in-law ALWAYS thinks she knows everything about everything. I CANNOT stand it. My fiance is a grown ass man(turned 21 in Feb.) but...When my fiance and I got engaged, she flipped. (he had to stay with me at my mom's house for a WHOLE WEEK). When he bought a new car, she flipped. When we started looking at houses, she flipped. When we chose a house and bought it, she flipped. When we moved in together she flipped. When she found out I'm pregnant, she flipped. Throughout everything, she tries to tell him what he needs, should/shouldn't do or buy, etc. When she found out I was pregnant, I was 17 weeks along and she questioned whether I really was or not and asked for proof. I'm 20 weeks pregnant now, and last week he told her about the baby kicking hard when I put my cold hand on my bump (he missed it because he was at work). ...This fucking know it all..."She can't feel kicking yet. She's just making it up for attention" I started feeling movement at 16 weeks (flutters) and decent kicks at 18 weeks. I really want to put her in her place (which means her pointy nose OUT of our business) but my fiance told me I'm not allowed to be rude to his family...yet they're rude to me (CONSTANTLY) and all he ever does is say "can you knock it off" and leaves, never actually standing up for me. I'm not even allowed to call them out.

What do I do? I can't deal with this stress and being the sassy bitch I am I feel like I NEED to stand up for myself (even if it might make him mad)....HELP 😩😩😩