If you believe criminal activity is involved, contact local law enforcement. If it involves federal crimes, contact the appropriate federal agency. ICE would not be my first choice for financial crimes.
If there is any violence in his house, he must move out and contact an attorney to have her removed from his residence. It would be a problem if he is accused of domestic abuse.
Your parents should suggest your brother have a complete physical and mental health evaluation. Just because someone makes a good living does not mean that they do not have physical or mental health problems - especially after a year of virus lockdowns.
Unfortunately, it is not unusual for a person to be taken advantage of by another person wanting to live in the US. While most visitors to the US are honorable individuals, there are horror stories. Your brother should be encouraged to meet with an attorney about his rights and responsibilities in his new relationship and living arrangement. Be sure the advisors point out to your brother that she cannot legally work in the US so her line of work is irreverent. A reality check about sponsoring a foreign national spouse or worker may shock him into action.
Other than offering a sympathetic ear and a place to escape to, there is little you can do.