“Nobody in your family was a swordsman, or used a sword?” asked Shichika. He had merely spoken his mind, but in fact, the question was insightful. The Mutant Blades of Kiki Shikizaki had one common characteristic, their venom, and that venom was so potent that just holding one made you want to kill – it made its own an assassin who lived to carry out its murderous potential. An exception the likes of Meisai Tsuruga, who had tried to use the venom to do good, only proved the rule.
Whoever they were, the Itezoras could not have been immune.
One thing was for sure, Swordsman or not, whoever owned the Twin – had used the Twin.

Katanagatari Omnibus 2, Page 231