Bachelorette Party Etiquette?

[soontobemarried]

I'm getting married and my maid of honor and one of my bridesmaids are super excited on throwing me a traditional get-drunk bachelorette party. However, my other two bridesmaids are my fiance's sisters and neither of them will be 21 (one won't even be 18!). I've already suggested having a low-key bachelorette party, like going to dinner and then maybe having a 21+ post party, or just having an all-girls sleepover, in order to include my future sister-in-laws; but my girlfriends don't seem that interested in that. What should I do?