Your sister said "Since you disassociated yourself".
That's right you knew the consequences to your actions. So what did you think would happen? That they would disrespect Jehovah is order to associate with you. Not if they want to remain faithful to God. Boo hoo now you want to be part of the family and be at the wedding. Very convenient.
Your sister adds: "you felt a lot of resentment, and I can't say that I wouln't have done the same thing. But I wish you didn't, and I pray every night that one day you'll take it back, and come back to Jehovah!"
That's not brainwashing dude, she's talking to you from her heart. I can tell her faith is strong, but in the end she'll be better off on Jehovahs side instead of yours. You have nothing to offer her but death.
Your sister then says "It's just one of the unfortunate consequences of the step you took, the life you chose. "
That's right she's telling it to you as it is. Now I know what you and your apostate friends will say "she's just brainwashed, blah blah blah". The same old excuse to everything a Witness says but the truth of the matter is that she knows what the Bible says and what Jehovah expects from her. You could learn a thing or two from your sister.