All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Jubayr Ibn Mut'im narrated that the Prophet said: “One who severs ties of kinship will not enter Paradise”. [Al-Bukhari and Muslim]
Besides, Abu Ayoob narrated that the Prophet said: “It is not permissible for a man to forsake his Muslim brother for more than three days, each of them turning away from the other when they meet. The better of them is the one who gives the greeting of Salaam first.” [Al-Bukhari and Muslim]
These two Ahadeeth mentioned above are clear evidence that a Muslim should not sever his relations with another Muslim more than three days. If the parents of a Muslim, or one of them, or any other relative ask their children to cut their relationship with Muslims, especially with their relatives, then they should not obey them. Rather, they are obliged to disobey them since a Muslim does not obey any one when it comes to disobeying his Lord.
The best way to unite the two families and bring them near to each other is to remind them of the severe punishment and consequences that the Prophet mentioned concerning those who cut ties of kinship; e.g. the forenamed Ahadeeth and many other Ahadeeth that warn against breaking relationships. Among those Ahadeeth is the Hadeeth reported by Al-Bukhari in his book Al-Adab Al-Mufrad from ‘Abdullah In Abu Awfa that that the Prophet said: “Mercy does not descend upon a people among whom there is a person who severs ties with the kinship.”
You may also tell them that among the noble qualities of Muslims is to be kind to one another and love each other as the Prophet said in this concern: “The believers in their mutual kindness, compassion, and sympathy are just like one body. When one of its organs suffers, the whole body responds to it with wakefulness and fever.” [Al-Bukhari and Muslim]
Finally, we thank our brother for his good efforts and ask Allah to help him achieve his noble goals.
For more benefit on the prohibition of severing ties with kinship, please refer to Fataawa 255448, 323599 and 361402.
Allah knows best.