What is the ruling of Sharia on a sister becoming pregnant while she was engaged, and then she was married in the second month of her pregnancy? And what should the questioner do regarding her father, whom she hates due to his mistreatment of her?

The mother has done well by enduring her husband's ill-treatment and striving in raising their children, and she will have a great reward. The father's bad behavior does not negate his right to dutifulness. You must associate with him in kindness and supplicate for his guidance. Strive to be dutiful to your mother and treat her well. As for your sister, as long as the marriage contract is valid, the child is his by consensus. We advise you to strengthen your connection with Allah and rely upon Him.