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Of course you're always worried about whether people get offended, but honestly, if they are your sisters, close relatives, and your best friends, they will understand the circumstances. The main thing is that they will all be standing up there with you and that's the most important part. I don't think they (or you) should ponder over the "title" of who gets to be maid of honor.
Pick someone that is very reliable as your Maid of Honor. That person needs to be punctual, organized, and EXCITED about your wedding. If they aren't, then you'll end up pretty miserable. They need to know how to get along with others and how to host parties and all that stuff. I mean, don't pick your MOH based solely on these characteristics but they do come into play because, after all, the MOH has duties.