Most etiquette guides say that the sister nearest in age to you should be your maid of honor. If you're close to your sis and she's responsible enough to handle the role, you can call it a day and move on to the cake-testing sessions!
If your sister is much younger than you or isn't particularly responsible, your decision isn't quite so clear-cut. Some brides aren't all that close with their sister(s) and would feel more comfortable asking a friend to fill the role. If you're planning to bypass sis, talk to her about it first. Chances are she'll understand, but you should be sure to include her as a bridesmaid.
Just because you don't have a sister doesn't mean that a family member can't serve as your maid of honor. Although we'd advise against dressing your brother in high heels and satin, it's become much more common to see men stand up on behalf of the bride. If your brother is already serving as a groomsman, you can always turn to a female cousin you're close to, or even ask your future sister-in-law to step in. Don't feel obligated to ask your fiancé's sister to be your maid of honor, although you should at least consider asking her to be a bridesmaid.