Hmmmm. You have a point there. I guess if really depends on it the SIL is a good sport and would not ruin everyone else's good time. I'd be amazed if Rosie actually accepted. Considering she is already 8 mo. preggers and they are just now sending out invites, one would presume that the event will be at least two weeks away (in my circle of friends, we'd have to send invites at least a month out as everyone is so busy). If Rosie went, she might well be giving birth at a theme park! I still think they could have tried to make it seem like Rosie's idea not to go rather than to straight up disinvite her.
Just because someone apologizes it does not meant at a) the injured person has to forgive (especially when it seems the apology is insincere and self serving) b) that it automatically makes everything right. The sister was cruel to him. Should she get a pass just because she is the sister? If an acquaintance had publicly humiliated him, would you think he should go to their wedding? SHE thought they moved on. That she does not want to put in the work to mend the relationship speaks volumes.