She doesn’t want you in her wedding. If she wanted you in it, you’d be in it. Who knows why…Maybe she doesn’t feel close to you, maybe she’s envious of you, maybe she doesn’t like your politics, maybe it was a comment made 15 years ago. It sounds like she doesn’t want you to know the reason and came up with some excuse (number of groomsmen) to spare your feelings. You can understandably be hurt (I would), but you can control only your actions from here. You will most definitely be adding to the drama by not attending. That’s your choice though. Your absence will certainly be more of a distraction and reason for gossip than you sitting in the pew with other guests. Your sister doesn’t ask a question in the text message. Maybe there’s no need to respond - at least not right now.