Otto no Osananajimi ga Dokudatta Ken

The protagonist, Miyuki, lives happily with her husband, even though they share the same property. However, ever since her husband's childhood friend, Ikumi, who has returned home, has been staying with her parents-in-law, she has been acting as if she owns the place. In her husband's eyes, she tries to be superior. What's more, she decides to hold a birthday party for Ikumi's son at her parents-in-law's house, even though he's not her child! The hellish birthday party begins... (Source: Comic Days, translated)