Sister-in-law secretly went back to her family, and my brother was so angry that he didn't want her. He just doesn't want my sister-in-law anyway. How can I persuade him?

Don't persuade me, it's not the same as divorcing a wife in the old society. The child is only five months old, and the court will not support it if you want to divorce!

If their relationship doesn't break down, they can calm down for a while and then make up. After all, it's not a particularly excessive thing. That's what people do. When they are unhappy, the more others talk, the angrier they get. Leave him alone until he calms down. The more you persuade him, the more he works, but the opposite is true.

If their relationship really broke down (then why have children! The baby is only 5 months old! It's no use trying to convince you. See what your sister-in-law thinks.