Will the original family members bear legal responsibility for insulting, threatening and harassing their third party by calling and sending text messages?

This is your friend's fault. Their insults and threats are a kind of anger in law, not a legal responsibility, but your friend has to bear legal responsibility, and he has to bear civil responsibility when he knows that the man has a family and has a baby without permission! You have to convince your friend that if it doesn't work out, you can negotiate with the man, make compensation, raise the children and divide his property!