Bachelorette Party Trouble- MOH Is 20 Yrs Old

Hey....maybe someone can help. My sister is getting married on Oct. 8th of this year. I`m basically planning every single detail of the wedding, finding things, ordering things, BUYING many things, doing lots of work on my own time....invitations, printing favors out on the computer, etc. Now the subject of a bachelorette party comes up & we have a problem. The bride & groom want to go out together, with the whole bridal party, to a bar or something like that. They want NO strippers, but they do want to get drunk. I said, since they just want a party together, guys & girls, no strippers or anything, why can`t we just have a regular party at the house so I can be there? They said NO party at the house because they dont want to do something they do every weekend.....As of now, they still plan on having the combined party, at a bar, without the maid of honor there. That`s really not fair at all but I dont want to ruin the WHOLE bridal party`s fun because I`m a few months short of being able to drink legally....:( What should I do??? Please help :(


Thanks,
Erika