I'm looking for manga where the character has/had an established love interest, but eventually falls in love with said love interest's relative instead.

Example:
Main character is a boy who is in love with a girl, but eventually develops feelings for that girl's sister instead
Main character breaks up with her boyfriend and eventually falls for his brother

It can be the central plot point or completely incidental (authors do that sometimes), both are okay with me.

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If I've made a thread in the "I'm Looking For..." forum, I am only interested in Japanese manga. I am not interested in non-Japanese media at this time.

DN Angel -- he eventually falls in love with her twin
Himawari -- spoilers by recommending this? (though it's mostly from the POV of the guy, but still an sweet oneshot)
Parfait Tic -- cousins?!
Majo wa Nido Aegu -- spoilers by recommending this as well? XD
Cross Game -- or so it is implied...
Moe Kare!! -- oh noes, she cannot choose between the two twins!
Bunbetsu to Takan -- one of the worst manga I've ever read, but it fits your request .___.
Kuragehime -- possibly? One day maybe? -slaps inner fangirl-

And there's probably a lot more I can't think of right now...

Turn A Gundam