Miel - Kekkon Seikatsu o Mamoru Tame ni Dairi Kozukuri de Netori Ana Yome ni Sareru Niizuma

The protagonist is the son of a wealthy and influential family, and he's always had a thing for his older cousin Nodoka. Just seeing her getting lovey with her new husband pisses him off to no end.Just then, the family head summons him, and tells him that he's fed up with that husband's inability to get Nodoka pregnant, and they want you to get her knocked up in his stead!
If she doesn't want her happy married life to end, she has no choice but to accept your seed, but along the way she might just become your loyal slut...