Forbidden Love Yaoi

Uncle x nephew
Daisuke moves in with and looks after his uncle with no life skills

Uncle x nephew
Nephew falls in love with his chess-playing uncle.

Sister's lover
Daichi has a bad feeling about his sister's seemingly perfect lover, Jun, but once they become friends he brushes it off, eventually falling for the amazing man. But Jun is not at all who Daichi thinks he is.
Trigger warnings: lying/manipulation, drugging, noncon, attempted rape.

Sister's boyfriend
High school student Ren finds his sister's boyfriend, Kazuya in his bed one day. Kazuya starts to play with Ren, while helping Ren deal with his unrequited love for his best friends, but both their feelings start to change. Seme sucks.
Trigger warnings: noncon, cheating, rape, statutory rape.

Sister's boyfriend
Nazuna has always been in love with his twin sister's boyfriend, Takumi, and now that she's passed he frequently goes to check on Takumi, even using his identical face to tempt Takumi to have sex.

Sister's husband
Yuki plans to keep his love for his deceased sister's husband a secret, but when Keiichirou tries to bridge the gap between them, Yuki is torn apart by his guilt toward his older sister.This story was so well done that it made me cry, and the sister character was actually well done and mattered, which is rare in a yaoi plot.
Trigger warnings: incest (by marriage).

Mother's boyfriend
A naive young master gets a little too close to his mom's boyfriend, who has an agenda of his own.
Trigger warnings: lying/manipulation, cheating, rape, statutory rape.

Best friend's father (unrequited)
In love with his best friend's dad, so he uses the best friend to stay close to the object of his affections.

Brother's stepbrother
Nagi transferred to a boarding school to look for his brother, but instead he meets Kou, who strangely says the same words to him as his brother. I'm honestly a little confused as to the relationship here, but it's not technically incest.

Neighbors (calls himself his "father")
When Akane was born to a single mother, his young neighbor Kyouji announced that he'd be Akane's father. Now, years later, Kyouji is still more than happy with that relationship, but Akane is not having it.