The Silent Sisters (A Lore Video!)

I would have thought so too. However, the Wiki takes it information from the original DAO: Prima Official Game Guide, which appears to contradict itself as initially it says that not all females of the Warrior caste join the Silent Sisters, suggesting that they are drawn from the Warrior caste, but then says that their caste plays no part in whether a woman is accepted into the order, only that they can prove themselves in the arena. Which seems odd to me as even Astyth the Grey was originally from the Warrior caste and the whole point of the DAO dwarven commoner origin was surely that they should not have been allowed into the arena as they were casteless.

So may be you can be a Silent Sister so long as you belong to a caste regardless of whether it is the warrior caste but you cannot join them if you are casteless. In that respect the Legion of the Dead would be unique.