How long ago did she set her date again? For what it’s worth, changing your date won’t set a precedent in the future. If you get pregnant before her, what is she going to do. She will be the irrational one if she is annoyed about something you more or less can’t control (like depending on fertility). But you can control the date, if you can use your deposit etc.
My brother is 35 and the woman he’s been seeing is 37 so I think all this is in the realm of possibility. I kind of think he wouldn’t even think or ask about getting married two months before me if I had the date set. Even by asking you’re hoping she’d say she was ok, right?
I’m not sure why I’d be annoyed but I would be. I think I’d be stressed out from wedding planning even if I had no role in his wedding. I’d get over it if need be but sure, I’d be miffed. (Also I think there might be cultural reasons that I couldn’t go to my brother’s wedding as it is too many good things—but that’s kind of silly).
And yet it kind of sucks if you really want a spring wedding, which I get? Just from an outside perspective, yeah, 6 months before or after does make things look better than two. Given the differences in the weddings I don’t think it’d be unreasonable to keep your date. But I do understand where the future SIL is coming from.