Could you please guide me in the following matter: My sister has three children.

Question

Could you please guide me in the following matter: My sister has three children. She is married to a Moulana who is a graduate of Dabel in Gujrat. He also completed his hifz in Gujrat. The said Moulana performed the marriage of his son (my nephew) to a lady who is a Taoist, who refused to convert to Islam and remains a Taoist. He claims that that such a marriage is permitted, even though we believe otherwise. This union produced a child. Last year he married his daughter to a Christian man who also refused to embrace Islam. Initially, we agreed to attend the wedding but after being advised to boycott the unIslamic event, we did not attend. This caused a major rift in the family. My sister felt insulted that we refused to attend her daughter's wedding and in response, she completely severed her relationship with all her siblings and her mother. After four months she made limited contact with her siblings through social media, but she has not yet come to meet her mother. Not even during Ramadhan or Eid. She and her husband, the Moulana, wants us to apologize to them, for not attending their daughter's wedding. The Moulana claims we created the "elephant" in the room, so we must apologize and fix that, before they are willing to make peace with us. Please advise on the following: 1. How should we treat my sister and the Moulana. 2. Should my mother go to visit her? 3. What is the Islamic status of someone who believes that such marriages are permissible? 4. How should we treat my nephew and niece who chose to marry non Muslims and regard it as permissible? 5. Are we allowed to socialize with them, to maintain family unity?