It's gotta be everyone of them have sexual hijinks gone wrong. Akari seems the fishiest out of the girls since her affection seems directed at the girls, likely Hinagiku the most since they have the closest relationship. She seems a bit too normal for how fucking deranged the sisters behave, especially around Tsukushi and she'll no doubt have noticed they're all horny AF for him. She could be a Yuri sexual monster with murder tendencies.
She could also just be what's presented, I think there's enough of a gap in her internal monologue for it to be possible.
There's almost definitely some kind of sexual relations with all sisters, probably to varying degrees. Original Tsukushi is probably the most guilty. We'll see.
Thanks for the TL.