Hi
A little difficult as there three Mary's married to Ralph Forster, 1849 -1850, Weardale, Teesdale area. However although I think to be certain you may need a birth certificate of one of the children to prove it, there was a marriage Weardale 1849 to Mary Addison.
In the 1851 census living with Ralph, Mary and Joseph is a son Jno Addison aged 5. Now whether Mary had been married before or Jno was born out of wedlock I'm not sure yet.
Evie