He is also now part of a showbiz family.
In 1992 he married Martha Ross, mother to TV presenters Paul and Jonathan, and herself an actress and performer.
This use of ‘to’ is here only a journalistic variant and has no significance. It’s really a way of presenting the good lady in question rather than the more mundane ‘of’.
Personally if I were the mother of Jonathan Ross, I think I’d keep quiet about it!
When “mother to” is used, or at least in the instances in which I’ve seen it used, it is meant to signify the action of mothering others when the “mother” is not actually a mother of those with whom she is interacting. As in:
“My friend David’s mom was like a mother to me.”
or
“The basketball coach was like a mother to her players.”
But if we’re matching mother and son/daughter, I think the best option is to use “of”: