I missed the part where you talked about withholding information from your SIL’s family. Please don’t do that. That’s extremely petty and that’s how “family wars” start.

I would still let your guests decide because a wedding is a much bigger deal than a baby shower. I would think most other people would say the same. I feel like a baby shower isn’t as big of event as a wedding. Most people are there to give stuff for the baby. I think going to a wedding is a different because it’s an honor to be a part of a couple’s special union sending. Plus people can have multiple baby showers (if they really want for their other babies lol) but a wedding is (hopefully and maybe mostly) a one time thing.