بسم الله الرحمن الرحيم
It is very sad that the said Muslim became a Christian. According to the Shariah now the man has become an apostate. It is unlawful for his family members to continue any relation with him. His children should also be separated from him and his mother should not accept any money from him. Also his money should not be included towards his father’s burial expenses. Those family members are right who are asking his mother, sister to cut any relation with him. The ruling pertaining to apostasy is very severe in Islam. We quote here a question and its answer related to the same topic from Fatawa Darul Uloom Deoband:
Question: A Muslim became Christian. What is the Shariah ruling about making him friend and hold him dear and meet him cheerfully and eat and drink with him?
Answer: The person who left Islam and became a Christian is an apostate. It is haram to have any relation with him. And it is unlawful to meet him and shake hands with him cheerfully and eat and drink with him. Those who met such apostate cheerfully and ate and drank with him committed a sin, they should do taubah and istighfar and avoid it in future. (Fatawa Darul Uloom: 16:400, Question:789)
Allah knows Best!
Darul Uloom Deoband