Traditionally, bridesmaids would include the bride's sisters and close friends. For some lucky brides, they know for sure who their bridesmaids are going to be even before the guy proposed, for others, choosing the members of the wedding party is a traumatic experience. This does not really answer the bridesmaid problem though. Sometimes the problem would be an overabundance of eligible bridesmaids that is nearly impossible to narrow down; sometimes the problem is a shortage of bridesmaids that it's nearly impossible to find anyone.
If you have a lot of potential bridesmaids, list them all down and pick out which ones are closest to you, which ones you have known the longest and are most likely to help you with everything from wedding jitters to potential disasters. If your list is too long, because of some people who are there because of obligation, simply remove them from the list. It is not necessary to ask someone like your estranged sister to be a bridesmaid if you are not comfortable being with her, but if you would like to make a reconciliatory gesture by asking her, go ahead. On the other hand, do not be guilt-tripped into asking someone just because you are expected to. Your wedding party should include the people you would like to share the happiest day of your life with, not the people you think should be included just for form's sake.
Before booking your friends and sisters for dress fittings, you should know for sure what you expect your bridesmaids to do, and what duties you would require of them. Do you want them to be involved with planning every detail of the wedding or just expect them to show up for the ceremony and reception?
If your bridesmaids are needed with putting the wedding together then you should choose friends and family members that are or can be available during the planning stage. You should also choose people that can work together or are comfortable with each other. Instead of helping, bickering bridesmaids would be more of a hindrance.
If the bridesmaids were just required to show up for the rehearsal dinner and the wedding, then the criteria would only include who you would like to have in your wedding party and who would be available for the wedding day. This might involve a lot more hairsplitting than if bridesmaids are expected to help out. If the list of potential bridesmaids is still too long, some of your friends or family members could be requested to perform other duties instead, like doing the liturgical readings or the offertory during the mass.
