As well as you know your silbling, do you REALLY know him/her?
Maybe when you had your simcha, something was going on in her life that she didn’t want ANYONE to know about. Maybe the siblings spouse found a tumor ch”v and they weren’t sure what it was. Maybe there was “mis” in the family and she wasn’t able. Maybe someone had to go the “M” that night and couldn’t promise.
Being dan l’kag zechus applies to sisters and brothers as well. May your sibling just can’t handle it. And if you could and all works out then TOVOI OLECH/OLECHA bracha!
I say, if you can, do it with a SMILE. Boruch Hashem people are getting married and simchos all around. Mazel Tov