Oh, wow... that was surprising... thought Mayuko was cheating on her husband and Haruka was NTRing him.
Haruka has fallen in love with Mayuko and is substituting for his brother Ayahiko, while Mayuko is in deep delusional state after tragic death of her husband.
From ethical stand, he is abusing a mentally ill... that's even more fucked up than cheating.
Hope he can fix her, but seems fine taking advantage of her state. Yikes.
Thanks for scanlation!