Your maid of honor plays a very important role in your wedding, from easing your planning burdens to keeping you calm in the midst of a wedding disaster. Since she'll spend so much time with you in these days leading up to the big event, it's a good time for her to get to know your family if she doesn't already.
When Your MOH is Your Sister
Since she's already a part of the family, this makes things really easy. Your sis will know the things that tend to make you and your mom disagree and she has plenty of experience keeping the peace between dad and grandpa without you ever saying a word. When your sister or another close relative are filling the shoes of the Maid of Honor, you may need to share some info on your future in-laws rather than your own family.
The Best Friend
Chances are, if she's your best friend and the only girl you would consider calling on to fulfill your MOH duties - she already knows most of the little things that make your family unique. She'll probably be working closely with your mom and sisters to pull off this perfect day so it's a good idea if your maid of honor is someone well liked by your family. Keep her up to date on any conflicts or issues among your family that she may not be aware of and, if your maid of honor is close to your family, involve her in resolving any problems that arise.
Future In-Law as Maid of Honor
Enlisting your fiancé's sister as your maid of honor is not at all uncommon, but can create some touchy situations if your families aren't really close. If the two of you have a good relationship and have known each other for a while, you should feel comfortable discussing any concerns you have with your family. Be careful not to share too much information though or you take a chance of really offending your parents and siblings.
Your maid of honor has a big role in your wedding and the days leading up to it. Whether she's someone you've known all your life or a new addition to your married family, she will be getting to know your family quite well over the next few months. Take the time to make her aware of your family's little quirks and, if she doesn't know your parents, make sure to relay a few tips that may make communication easier - but don't tell more than you would want a stranger to know about you!