like Allegro Agitato where the girl likes her mother's boyfriend and Mama wa Doukyuusei where the guy likes his father's girlfriend.

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Incest♥Necrophilia♥Selfcest

Unworthy Son is all I can think of at the moment...

This category might help, although I think it's pretty neglected seeing as nobody's added any of the series so far mentioned:

http://www.mangaupdates.com/series.html?category=In+Love+W ith+Family+Member%27s+Lover

Also, at the start of Honey Hunt, there's a situation quite like this, although it only lasts a chapter or two.

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世界のどこかに 必ず
キミの居場所が ある筈

G Senjou Heaven's Door has a similar situation.

Honey Hunt sort of fit for the first couple of chapters

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Help! I'm looking for... manga with dangerous guys http://www.mangaupdates.com/showtopic.php?tid=39874&pa ge=1#post594033

Majo wa Nido Aegu has it, I think. It's been a while since I read the chapter summaries (lol) but I believe she had a crush on one of her mother's former lovers (her mother was a very famous hostess, as I recall) who turns out to be her main love-interest's father and also seems to have an interest in her. Lol, that sounds so confusing.