Bismillah,
Asalamu Alaykom,
It is not allowed for him to force you to cut relations with those who it is wajib to keep relations with such as parents and siblings. If you cannot solve an issue by speaking to him, then it's advised you get a trustworthy scholar from the community to mediate and advise him that this is haram and a big sin. If you don't fear any harm to yourself, you may also contact your family without his permission
May Allah grant you a