Given the importance of maintaining family ties, can one refuse to attend family functions due to the fact that those relatives declare the Shi'a as Kafir?
We should not cut ties with any relative even if they are bad to us and call us Kafir.
Our behavior should prove to them and other relatives our real identity and whether we are Kafirs or not.
Ahlul Bayt (AS) taught us to mix and treat our relatives and tribes with best manners. People wll then know the real teachings of Ahlul Bayt by seeing the good practice of the Shia.
Wassalam.