Who should be the maid of honor?

You could have both simply be "attendants" or "bridesmaids" and not have a maid of honor, and delegate different responsiblities to each of them (one handles day-of, the other handles bachelorette party, etc. etc.) so they don't have to work together. Or have no attendants at all!

ETA: You also could ask your friend to be your figurehead MOH but explain the situation to your FI's sister, and tell her you'd very much like her to be involved in the planning, that way if your friend doesn't live up to your position you will still have help.

I am leaning towards asking them to both just be bridesmaids and giving them different tasks... however, I''m not sure that the best friend would be good at/ be happy about doing ANYTHING.  And I think the fact that I did not specifially ask her to be my MOH would make her really mad.  I''m so torn!

Do you know your fi's sister well enough to confide in her about the situation? It's possible that she could completely understand.

I don't think there's any harm in having a non-involved MOH, unless you really think that you'll need a lot of help. My MOH lives (literally) on the other side of the country and has 2 young daughters, so I'm not expecting (or asking for) any planning help from her. I want her to be my MOH because she's played a large role in shaping who I've become, and will forever play a large role in my life... not because of the role she'll play in planning my wedding.

Then again, that's not the best way to go if you want a MOH to play the traditional role--planning almost as much as the bride!

I think if it were me (working with very limited information here) I'd ask your friend to be your MOH, and your explain to your FI's sister the situation. It's not worth upsetting your friend even more when she's going through a rough time.

Then again, I'm a "make everyone happy" type, so I'm sure a LOT of PSers will disagree with me though!

I *would* just ask my best friend to be the MOH (to avoid hurt feelings) and explain everything to the FI''s sister so she would know that I''d  really love her help... but I''d hate to ask the best friend only on the principle that I''ve known her longer.  We used to be very close but lately our styles/lives are different and I get along much better with the FI''s sis.   

It sounds like you''ve already made your decision Only issue is how to break it to your friend, but I''m afraid I''ll be no help there, I never know how to deal with situations like that.

I had a similar situation when deciding who to pick for what.  I have three friends that I'm very close with, one that I've known since high school, another since college and another since before law school.  Neither the first nor the second live in my state.  So part of me thought go w/the one from high school, I've known her the longest and certainly, we have a deep bond.  However, we're very different and some of me questions if the time we've known each other is part of what holds our friendship together.  So I also thought about person #3 b/c she lives here.  But I haven't known her as long as the others and we're also pretty different in some ways.  So person #2 is probably whom I have the strongest connection in some ways.  We're a lot alike and she recently got married, did not have an official wedding party but I was the "unofficial MOH".  She didn't want to hurt her sister's feelings or any other friends so just decided to have a "Wedding Task Force".  That was fun and nobody's feelings were hurt.
But what I really wanted was to pick her, even though she's out of state and probably couldn't help much, but she best understands me and is in a fairly similar position in life.  So I lamented over it for days and finally, w/my FI's advice, just decided to do what I wanted.  It's my wedding and it's about who I want as what.  So that's what I did.  I asked the other two to be bridesmaids and her to be my maid of honor.  I don't know if the others feelings are hurt but I sure hope not.  Since then, I felt great about my decision and to this day, although my moh can't be around to help, she's the one who's provided me the moral support w/things and the first one who went to try on bm dresses.  To me, choosing a moh wasn't about what tasks she could provide for me, but who she is and what she means to me.
So, with all that being said (sorry so long!), I'd pick your FI's sister b/c it sounds like that's who you want.  It's your wedding and you should be happy!

this may not be your situation, but i know a lot of friends who are getting married that feel obligated to put their fiance''s family members as their bridesmaids and regret the decision because they ended up leaving someone they wanted to have as a bridesmaid left out.  if this is your case, i wouldn''t choose your fiance''s sister.  go with your heart.  i would choose your best friend.  she probably knows you the best and has been there through thick and thin!  i''m sure she is not upset about you getting married, just that she wishes she could be as happy as you!!!!