Simon is right you can't make people not shun or make them associate with people they don't want to. That's pretty obvious.
What the state can perhaps do is deny privileges and benefits to religious or other organisations that promote shunning family members. It is then up to the organisation to either reform their policies or suffer consequences. This might include loss of charitable status, and denial of privileges, legal status or avenues of influence accorded to religious bodies generally. This does not curtail anyone's rights but nudges organisations in the right direction. In the case of JWs it would be no more than an invitation to live up to their own statement that:
No one should be forced to worship in a way that he finds unacceptable or be made to choose between his beliefs and his family.