My sister me her father within the last two to three years. She met him online, after doing a search on Facebook. Our mother and her father were never married. She has only 'talked' with him online. He has never paid child support. She is 27 years old. Can my sister or her mother sue him for unpaid child support. We live in Canada, he lives in the UK.:)

Was a child support order ever awarded? Did the mother ever even apply for support?

The mother ( not the children) can go back after child support if there was a child support ordered issued by the court previously. ( and only to limited amount after a time frame)

So if the mother had already when the children were minor children, got a court order, they can go back to that.

The children have no rights to collect since it was not ever their money, it was the mothers ( parents) money to pay for their being raised