ADVICE SISTER ALISON'S TAKE #1 (head):

You want to back out now, when the wedding is just around the corner?  That's definitely not supportive.  I'm surprised your sister asked you to be her honor attendant, and I'm surprised you accepted, based on your self-described less than harmonious relationship. But family is family, and your  sister obviously thought enough of you to give you this special honor. What I'm wondering is why you didn't speak your mind sooner?  If you had, you could have both worked out your expectations and cleared the air. Backing out now will just cause your sister more stress. She might have friends willing to take over, but it would be challenging for someone else to fill your shoes (figuratively) as a sister, not to not to mention your dress (literally) at this late date. 

This wedding isn't about you, so don't worry that you are far away and that you are missing out. In fact, it's  your sister who is missing out on having a sisterly helping hand, nearby. You can still connect, however.  Keep in touch with last-minute details through email, text messaging, and phone calls. Encourage your sister to include you in decisions when she needs help. For example, she might take photos with her cell phone of those flower arrangements, send them to you, and  then IM (instant message) you and the two of you can text or call to share your  thoughts. Just remember:  the decisions are hers so break your bickering pattern and be supportive. The bride is always right (and even if she isn't,  the decisions are hers to make).

ADVICE SISTER ALISON’S TAKE #2 (heart):

There is more than one "should" here. Your sister might have felt she should invite you to attend her, but if she really didn't want you to do it, she shouldn't have asked. You might feel like you should support your sister, but apart from the obligation, the decision to be honest and not attend her was yours--you could have said "no."

I think there's probably more sibling interaction issues here than you wrote about. In fact, your sense that you two don't get along could be one-sided. However, you don't have time for Dr. you-know-how to come by and put it all to rights. Do the right thing and go for the maid of honor title, and  don't  stress out (or stress her out) about it.  Despite your differences and an apparent wealth of friends, your little sis still chose you as her honor attendant. I'd be proud, if I were you.

The distance is a bit of an issue. You can't run errands and provide that kind of day to day support, but as an older sister and someone she trusts, you can be there to lend an emotional hand.  Encourage your sister to lean on you and be an impartial judge when asked. Assure your sister that you are truly happy for her and that you will be her "rock" on her special day. Then, follow the suggestions in Take #1.

When you do arrive for the wedding (get there a few days early to help out in the last-minute crunch) be friendly, helpful and cordial to everyone. When the wedding day arrives, you get to assist the bride (that's your special honor). No matter what how inconvenienced or distanced from the planning you feel, you are not the star of the show, the bride is. So step back and enjoy the journey, as you watch your little sister, shine!