I am descended from John and Martha Chambers through their son Joseph. The family lived in Allegheny City (now part of Pittsburgh). John was born in Pennsylvania and died in 1861. Martha was born in Ireland and died in 1868. John's estate documents identify one of his daughters as Jane and states that she was married to Thomas Bullock. The documents also state that Jane had died sometime after John's death but before the estate was settled in 1869. John and Martha also had a daughter named Mary who, according to the estate documents, was unmarried at the time John's estate was settled.
I had not been able to determine what became of Mary Chambers. She is not buried with her parents and she does not appear in the census in the Pittsburgh area from 1870 onward. After reading your post, I am convinced that after Jane died, Thomas Bullock married her sister Mary. Yesterday I found Thomas and Mary Bullock in the 1870 census in Nebraska. The two children living with them match the names and ages of two of the children which Thomas and Jane had according to John Chambers' estate documents.
One discrepancy I can see is that the age of Mary is a little bit off. The Mary who was the daughter of John and Martha was 16 in the 1850 census so I believe she was born about 1834.
Let me know what you think. I can send you the estate documents I have so you can decide for yourself if I'm on the right track.