Yandere

So the twins' parents died because they were shot and lit in fire by the uncle (dad's lover and cousin) because he was jealous of the mother..

Basically the story goes like this..

Younger twin : I hate my uncle but I had sex with him so I can have sex together with my older twin

Cousin : I hate my uncle too, but I came to foursome because I wanna have sex with Ren (the older twin)

Uncle : I only love your dad, but some crazy women who I fucked fucked my dear lover, which was how you guys

Vol 16. - Erotic ni Kojiakete
Vol 17. - Kemono na Kouketsu Egoist
Vol 18. - Kedakaki Kemono no Territory