The eleventh brother would be the Twi’Lek inquisitor but that isn’t the official name

^He could also have been the ninth brother.

Well, we have a Ninth Sister already, but as Third Sister and Third Brother show, the numbers can be duplicates. The Twi'lek Inquisitor could be the Eleventh Brother, but he could also be the Third Brother, or First, Second, Fourth, Seventh, or Ninth.

^The first inquisitor is the grand inquisitor, I think.

That could be possible

I think it's clear that there's one Inquisitor per number, and the gendered sibling term is just determined by the individual. The Third Brother either preceded the Third Sister or, depending on how Kenobi shakes out, could possibly be her replacement.