fmil wants to marry her son

hi guys, need some input, am I overthinking this?

For the most part, my (26f) fmil and I get along well. However, there have been a few snarky remarks made to me over the years that kinda stick with me bc they always come out when we are drinking. I feel like it’s the old “drunk words are sober thoughts” thing, so they’re kinda hard for me to brush off.

A new one popped up over the holidays at a surprise birthday party for FFIL. It was the end of the night and there was just a small group of close family and friends left chatting, all of us having had many drinks at this point. FMIL was telling a story she has told many MANY times before about how when my fiancé (26m) was a toddler he told his mom he wanted to marry her (she loves this story, I find it really weird that she tells it to me all the time). He was obviously a little kid and just meant that he loved her, and she told him she was married to his dad. Whatever, weird story in my opinion, but she loves to tell it because it probably makes her feel important, idk.

She usually tells this story and we move on, but this time she turns to me and says “just remember, FDIL, you might be marrying him but I was his first choice”. I was literally STUNNED, and it sobered me up IMMEDIATELY. One of the family friends gave her a weird look and basically told her she shouldn’t say stuff like that to me but she laughed it off.

This spiraled into a huge fight between me and my fiancé for not defending me, and him telling me he thought it was just a weird joke. I was furious that his moms friend defended me and he didn’t. We talked about it a lot and he understands these are territorial remarks and I need him to shut her down when she does this. He is happy to shut her down, but we haven’t come up with a good plan for how to do this without causing a scene.

Would love advice on how fiancé can respond in these situations without making it awkward? Or just advice in general because I’m sure this will not be the last time I hear something like this.