Kate I'm sorry if I came off rude, I didn't mean it that way. I just meant that even if your sister throws a hissy fit, if you both get married in the same year, it would ultimately be up to your parents to decide how they split the money. Your wedding will probably be a lot less, so even if you are only getting half of what you would normally get, it will probably still be a huge chunk for you to pay your wedding. If your sister is going through some thing where she needs her own special time, that's understandable considering the fact that she is a twin.
I think when the time comes that you are both engaged you should emphasize to her how exciting you think it is that you are both getting married, and that you will both get to do fun wedding planning together. Also, make sure you tell her how she'll get her special moment since she will have a big beautiful traditional wedding in Boston and you will have your DW. This way she doesn't feel competition. And if it makes her feel better to think that her wedding is somehow going to be better than yours, then let her have that piece of mind. In the end you will both have beautiful weddings.