I am sorry to say but your parents should have outright refused to let her be with her husband when he started to become violent instead of making things normal between them. Please end this marriage for her sake. She is in trauma and her children who might have witnessed their mom getting beaten are in trauma.
Get a lawyer and see how you can get her out of this marriage and put her jerk husband behind bars for DV. Encourage her to seek therapy because she needs it. Give her the confidence that her family has her back and to not think too much about what society says.