've read a manga a while back. i've only read volume 2 of this manga and it was about a girl living with a boy then the little sister of the girl came and lived with them. the sister took the guy to a "date" to spite her ex-boyfriend. after that, they all went to the beach and then the boy saw his old gilr-friend and then the girl living with him gets jealous.
I think there's a word "Love" OR "Affair" on the title...
That's all I can remember...

Sounds like the shallow series Pastel to me.