Mokoto is the current leader of the school. Mokoto's sister was the strogest but stopped to get married.
Setsuna is a student learning from the long haired teacher.
I don't know about Konoka's dad but it is to assume he was powerful and stopped to get married himself.
Setsuna met Konoka because of the Aoyamas (when Mokoto's sister was still leader. Konoka's dad was married and thus stopped swordsmanship).
Tsukiyomi does specialize in Shimenryu and is currently a mercenary. She uses two swonds instead of one. So does Setsuna's teacher