I think the NPC idea is a fair option, especially since it seems like we're all doing single characters thus far. It would give an option for still including her, without upsetting that balance of play. There's nothing to stop you discussing what his sister would/would not do with Tergonaut in PMs, so there is still that level of control over the development of the character.

Tergonaut has reminded me that IC posts are to wait until all six slots are filled, so I am retracting my earlier statement. I'm afraid I was a bit overzealous about getting into a roleplay again, and jumped the gun. I personally have no doubt that the remaining slots will be filled quickly.