You weren't rude at all- you were very nice to go where the other actual B-day person wanted to go.
If you want to be the nicer person, tell SIL "We couldn't have gone to XYZ without you; how rotten- to have your family go to your fave restaurant while you have to sit at home. We're saving XYZ for a special ocasion when you are with us."
For their next B-day or anniversary or other special date, then, go and do try it if they ask again (unless you really can't afford it, then no!). Most places allow at least some substitutions to menu items, so order what you can eat and remove anything you really don't like (glazes, sauses, cheeses) . See if you can find a dessert to rave about.
I know it isn't your thing, but if you are close to SIL it is a kind thing. If it's their special day, let them choose, especially if they are good the rest of the year with your dining choices.