Past-- Hero and Heroine were a thing but Heroine marries hero's brother when the hero refuses to marry her. (Hero has a wife he is separated from both don't love each other but she is not divorcing him and he is also not pursuing divorce actively.
Present -- Hero's brother dies, heroine is pregnant and now hero wants to marry her.
Reason -- Hero thought he was infertile cause he and ex-wife tried for children, never got any so. But he gets to know later that heroine was pregnant when she left him and she miscarried and his brother was there for her all that time and she genuinely started loving his brother. Heroine finds that hero's brother never told her hero had written to her.
Sub plot -- Hero has another brother who is physically handicapped but still want to bang bang the nurse (if I am not wrong)
P.S. -- I may be wrong about some details as I read it long back. It is one of those really old OTT HPs but I had fun cause I love angst. It's based in U.K. I guess.