So, what do you do when you have two or more special ladies in your life to choose from? If they all have the skills required of a Maid of Honor, then you always have the option to choose more than one. Divide up the duties so that each will have equal responsibilities - you may actually benefit from having two ladies at your side to help during the planning process!
If you insist on having just one Maid of Honor, choose the person who you feel can best fulfill the responsibilities associated with the role. Understandably, this can be a tough decision as those not chosen may be offended. In that case, you may lessen hurt feelings by choosing a sibling closest in age.