Wa 'alaikumus salam wa ra'hmatullahi wa barakatuhu wa maghfiratuhu. Your aunt seems to have fallen in the hands of a bad husband. That's why women should know their husband's character before marrying. Your aunt and his husband both need to change their character. Your aunt has to be stronger and her husband needs to be softer. You mentioned divorce is not an option. But why not? I mean if your aunt has financial backup then wouldn't it be better to leave this husband? He is ruining your aunt's life. Some people never change. Remember fir'aun? He was one of the most evil men. His wife was Hazrat Asiah radiallahu ta'ala 'anhu. She tried to change fira'un's character but it was of no use. He still remained that way and died as a kafir. From this story, you can see that the same case is with your aunt and your husband. With my limited knowledge, I see that either of these two action will be most apropiate here -
Tell your aunt to divorce this man. She should take her daughter with her, because most likely her husband will not take proper care of the girl. After divorce, your aunt should come back to her parents if possible, and if not she should live with her relatives. If even this is impossible she should try to marry a good muslim husband.
Your aunt should try to tolerate her husband's rough attitude and she should try to change his behavior. She should keep telling him that he is going astray. Tell him the consequences of his actions. Also she should find someone whom your uncle respects and tell him/her that the man is doing unlawful things and ask him/her to help. If this not possible then try to take this matter to an islamic court. If even this is impossible then your aunt is left with the only option to seek help from Allah. She should cry to Allah and ask him to change her husband's behavior. She should recite durood all the time with wudu. I think that option no.1 is better. By the way, you and other family members should support your aunt. Her husband should be punished for sleeping with other women. He deserves to be stonned to death if he really slept with other women, according to shari'ah. However, your aunt may choose to forgive him, this way he might be changed. I don't know if this is right or not, but I see that most of the problems in marriages is because of sexual dissatisfaction. So, if your aunt's husband feels the need of more women to satisfy his needs, he can marry three more women and then have more sex lawfully. If you need more advice, just tell me.