This happened in my family once. Everyone was shocked, which I think is understandable as it goes against ‘tradition’ and you naturally want to be there for close family members special moments. However they got over it very quickly and now it’s part of family lore but not a big deal.
You feel how you feel so yanbu for that, it’s natural to feel something when finding out news like this. It might just be the initial shock. Once you’ve had time to process it you may see things differently.
YABU to feel entitled to a wedding to go to though, a wedding is about the two people getting married and no one else, how they choose to go about getting married is up to them. Getting married isn’t a big life event for everyone. For some, especially two people whose lives are already intertwined via living together for many years and having dc, it might just be something that needs to be done. They may not have the time, money or energy to organise a ‘proper’ wedding but still want to get married.
The reason for their decision will almost certainly be nothing to do with anyone else, or a reflection of yours or anyone else’s relationship with them. It will have been a completely personal decision that they decided was best for them.
It’s not worth having a big family drama over what is just one day out of many. They’re married now, they can’t undo it. You need to come to terms with that and be happy for them. Vent to other family members who may feel similarly if you need to get it out of your system but don’t say anything other than nice congratulatory things to your sister.