Marriage Issue
Brother Ali. Your story is fairly common in Pakistan except two brother and two sisters situation. To my knowledge of Islam, you must always respect and care for your parents, but you never take their side when they are doing things against the teachings of Islam. We all must take the side of right (as described in Quran) regardless of our relationship. This is the only way we can eliminate sins from our society. Judge the conflict between the two families strictly in the light of Allah’s teachings and take the side of right regardless of consequences. Trust Allah when you are doing the right thing. Tell your parents respectfully in the light of Quran why they are wrong. Tell them over and over again if necessary. If still they put them in such a situation to cut off from both sons, then it is their free will and they should be prepared to handle the consequences. Still never disrespect them and always try to do everything for them within the limits of Islamic guidance.