Father forcing brother to marry unknown girl, is it legal to do so?

In the month of April of this year, my father forced my younger brother by threatening him to marry a girl and my brother agreed to do so on a condition to undergo marriage after 2-3 years. But, later on, we came to know that my father has illicit relationships with the mother of that girl. Moreover, my father use to go to their house on a regular basis. And now, he is forcing my brother to marry as soon as possible with that girl. I have to go abroad for higher studies , however my father is harassing us by saying that he wouldn't pay my fees unless and until my brother marries that girl. Even, my father is forcing my mother to not to talk with my brother. We are suffering from last 6 months. What should be do?