While I always say that you don’t have to do anything you don’t want to do, it may be easier on you to ask your groom’s sisters and sister-in-laws to be your bridesmaids.
If they are assuming they’ll be in the bridal party and mention it, you may have a difficult time explaining -- without hurting their feelings -- that they’re not being asked. Especially if you’re planning on having a long list of ‘maids -- five or more.
For brides just asking one or two people to stand as their maid or matron of honor, you can explain to your future in-laws that you’re keeping it small and not having bridesmaids just your own sister(s), closest cousin and/or BFF.
In the case that his relatives are not officially bridesmaids you may still want to invite them to the bachelorette party. After all, they will be your family soon.