The cast of Kaitei mikan no kawa mairu - 2009 includes: Kazuo Hara Hirosumi Higuchi Saki Nagai Mirei Taniko

Mahira Khan Askari goes by Mena, Maria, and Mairu.

Shizuo doesn't have sisters, he has a brother named Kasuka. Izaya has twin sisters named, Mairu and Kururi and it's episode 25

Because they like each other... a lot.

Mairu and Kururi are strange. (Even in the novels) Izaya comes out and practically says it was his fault that his sisters are screwed up. (not that he cares for them)

In the light novels, they kiss a lot ...and occasionally grope each other.

Mikado Ryugamine, Masaomi Kida, Erika Karisawa, Izaya Orihara, Shizuo Heiwajima / Shizu-chan, Kasuka Heiwajima / Yuhei Wajima, Kururi Orihara, Mairu Orihara, Anri Sonohara, Walker, Kyohei Kadota / Dota-chin, Namie Yagiri, Seiji Yagiri, Mika, Simon Belkzev (I think I spelled that right), Saburo, Haruna, Saki, and I don't remember the few other characters names.