Help! Controlling mil won’t let us move!!

I’m posting here bc I honestly just don’t even know what to do about this anymore. I’m seeing my mil tomorrow at my nephews birthday party and I know she’s gonna start something with me. My dh and I have decided to relocate across the country with our son. We have a lot of good reasons for moving, but his mom just won’t accept it and basically thinks we’re not going. He already has a job lined up in his field and we are planning the move in January. We have been planning for months and every time anyone even mentions it she just says “you know the rules. No one is allowed to move.” On my sons first birthday, she got into a huge fight with my husband before the guests came and screamed at us like we were children. We just visited the area we are moving to and my husband posted about it online. Well she texted him right away saying we can vacation there all we want but he’s not moving. Like what??? And before we left she called me on the phone and said the same thing. I just said “okay” because I didn’t even know what to say! She even now has his cousin replying to his Facebook post trying to tell him only to vacation there? His other family and my family are supportive, but she is just living in denial. Btw my husband is 32 and I’m 26 and we have built up savings and planned this whole thing. It’s not like we’re two 18 year olds moving on a whim. Tomorrow Is my first day seeing her since our trip out there and I KNOW she is going to confront me and say something at this party. I am tired of not sticking up for myself, but don’t want to cause a fight at a party. What should I do if she confronts me? Btw she is usually pretty nice and supportive like she’s not evil or anything, just always gets her way and controls the whole family if that makes sense lol. They’re a very tight family which I’m not used to.