the main character likes her friend or sister's boyfriend. I've read mangas like Missile Happy and Koi Suta and also Sprout. I've been looking for some more of those types of mangas. Does anyone know any more? I would really appreciate it if you did! Shoujo/Josei preferably thanks. 😀

EDIT: Thanks for the suggestions! Do you think you could also name me some mangas where the boy already has a girlfriend? Doesn't have to be her friend or sisters boyfriend, just that he's already taken. Sorry to be a pain >_<

Allegro Agitato - the main heroine likes her mother's boyfriend

i couldn't find anything else.

Third story in Anata Nanka Iranai is all I can come up with, apart from Allegro Agitato, as cztery said.

Current avatar features Badou from Dogs.

Papillon - Hana to Chou
Crimson Hero
Goong at the beginning he already had a girlfriend
Random Walk

♪MONSTARR~ will eat all your cookies and steal your bishies~♪ Φ_Φ

You could try Alice 19th

Would you like to join the Axis Order?

Second Random Walk and Allegro Agitato... Lady Georgie has that in the beginning, also Revolutionary Girl Utena (only partly, though), Anastasia Club.

Oh, and definitely Tenshi Nanka Ja Nai! One of Ai Yazawa's older works, the second-main couple features girl in love with guy who has longtime girlfriend and can't make up his mind...