Maybe they are sisters. Sena is about as old as Kodaka, so perhaps their old man decided he can't take care of both a son and a daughter with his irresponsible lifestyle. So, he let his best friend adopt the girl. With Kodaka already around and older, Kobato wouldn't have been a problem any longer.
Regardless, Kodaka did receive some awesome fanservice. Despite everything (like sharing the bed with another dude) he must feel lucky they ended up staying overnight.
But yeah, a jolly good episode. I like little details like Kobato behaving (and looking) normally during the somewhat formal visit to the Kashiwazaki household. Other than that visit, I think the whole long horse demon scene was great.