Sister doesn't seem to care much if I may not make it to her wedding?

My boyfriend (6 years together, long time I know…) has his oldest sister who is getting married next year and had the date set for a long while which is labor day weekend. We’ve already confirmed and said we’d be there several months ago.

My oldest sister just got engaged and I did mention to her that I’ve already confirmed to go to my bf’s sister’s wedding labor day weekeend already to give her a heads up of what was going on. My cousin also mentioned to her that she already had two weddings she confirmed to go to that weekend as well. I don’t mean to make it about myself but I would hate to have to choose and be split up from my boyfriend especially when I told his sister I’d be coming…

My sister’s response kind of shocked me when she said, “Well, whoever can make it can make it if it is on labor day weekend.” and she mentioned she may even do a destination wedding. She hasn’t set the date but it would be exactly a year (labor day weekend) from when her fiance proposed. I was really upset that she didn’t seem to care if me and/or my cousin wouldn’t be able to go or make us even choose when we told her ahead of time to give her a heads up when she hasn’t set a date. Is it really selfish of us to think this way? I would feel like I’m choosing but I guess if it has to be…then my bf could stay with his sister for the wedding and I go with my sister if she chooses to have a destination wedding that same day but I’d be a bit “salty” inside.