Kaa-san Aishiteru ~Jinsei o Kaketa Hahaoya Kanraku Daisakusen~

Haruki loves his mother Yuuko so much that he wants to marry her.

"You need to courage to be strong enough to come take me from your dad. Women like that kind of stuff, you know."

Just a friendly everyday conversation, or so she thought. But Haruki treated it like a promise, and as he grew, he waited for his chance.
His mother is the only woman for him -- and so begins his plan to throw it all away and take her for himself.

[From DLsite]