here's my opinion, just gonna lay it out:
i think your sister is being a bitch.
1. if they're going to be seperate months, what's her problem? the fact that your getting married first? big deal! maybe she should've planned hers for june, but you should tell her it's not a race to see who can get married first.
2. she really needs to take into consideration that you will be teaching that fall. you're going to want to have your new name to start teaching with, not go and have to do that after teaching for 2 years.
3. if she hasn't even started planning, then maybe you could just be like 'oh, do you want to plan together. we can help eachother look for stuff together...' or something along those lines. try not to let her get to you, and try to be as civil as possible. i know this is harder than it sounds, and she'll probably try to make it sound like you're trying to be a bitch, but in the long run you'll know that at least you tried to be as helpful, and as nice as possible in the situation.
4. i really think it's messed up that she's throwing the whole 'i'm not part of the family because....' thing at anyone. that's just ridiculous. and it's fucked up that she's even trying to play those mind games. next time she says that, just be like 'don't go throwing that at me or anyone else. what's your real problem?'. put her on the spot, turnabout is fair play. she wants to put you on the spot, do it to her, just in a nicer way, you know. don't be all mean and nasty, take a deap breath even, so you sound calm and collected, even if you are wanting to just rip your hair out at that moment.
most of all, and remember this, ok:
DEEP BREATHES, everything will eventually work itself out.
*hugz*