AITA for not allowing my future MIL to have her plus one stay at the Airbnb?

REPOST: I deleted my other post as I realized I posted with my main account that shares a username with almost every account I have that is connected to his family and my own. 😅

I have to start with some back ground. I (28F) and my fiance (30M) got engaged 3 years ago together for 5 years. Our engagement was extended as we both wanted to finish our masters degrees before having our wedding. This has given us a great deal of time to plan and pay for everything on our own.

About 2 years ago, we went to a family event for my fiancé's family. This was the first time I got to meet my future mother-in-law's best friend Sherry. Due to past habits from growing up I am more of a wall flower who observes quietly. During this event I watched Sherry (who I believe is around her late 40's possibly early 50's) flirt with not only with my fiancé but also his 2 other brother's especially his 16 year old brother. I notice my fiancé become uncomfortable and ignore her advances towards him and join his two sisters across the yard. By the end of the event I was very uncomfortable. I didn't say anything at the event as it was my first time meeting Sherry and wasn't sure if I was reading into her flirtation to much as I know some people can be overly touchy and friendly withour knowing.

At the end of the party while driving home I brought up the topic to my fiancé. My fiancé immediately dumped that Sherry has constantly hit on, touch, and flirted with all 3 of the brother's and even has slept with the oldest. He explained that only the siblings believe him as the rest of the family brushed it under the table. As we were driving home, my fiancé mother texted asking if we were going to give Sherry a save the date for the wedding as she should be invited. We discussed it and both agreed we didn't want Sherry at the wedding. We informed his mother that we would not being inviting her.

Fast forward to earlier this year. We have attended many family events where I have witnessed the same flirtation and touching from Sherry to my fiancé and his brothers. This last one, my future MIL told us infront of Sherry that Sherry is her plus one to our wedding. I looked to my fiancé and he smiled without arguing. On the drive home, I asked about the change. My fiancé said it's not worth the fight. I told him I would follow his lead.

For our wedding we booked an Airbnb just for the wedding party as a lot of our wedding party lives out of state and some of his family as its decently far for them (5 hours). We have paid for everything with the airbnb. My fiancé and talked and agreed his mom could stay at the Airbnb but we are both uncomfortable with Sherry staying and informed his mother of this decision. She was upset and demanding Sherry stay at the Airbnb. We gave her all the hotels near the Airbnb maybe 6-7 blocks away. My fiancé is exhausted and frustrated with his mother's behaviors and I feel that I am the one to blame for us putting our foot down about Sherry. Do I just let her stay at thr Airbnb with is and the wedding party and deal with the uncomfortably?

Sorry if this was long or there is any bad punctuation this is my first post ever on reddit and I'm just distraught for putting this headache on my fiancé.