I believe the option to have sisterhood as your clan tag is full. You can still join and be part of the clan. There are alliance clans you can join to have a clan tag.

[quote]ATTENTION LADIES: CLAN TAGS are always maxed out (only 100! Srsly bungie!) which is what you apply for when you click "apply to clan". But the GROUP is always open and has room. It's Bungie semantics. You are still considered a 'clanmate' and participate as a full member by being in our group, you just don't get a nametag right away. We have alternate clan tags you may use once you become a member. Please apply to the GROUP not clan.[/quote] Message from admin, who states it better :)