Hi there,
Thank you for your suggestion. I have been unable to find a marriage for any of the three brothers, and more than that, they all declare themselves to be single when they fill in the census. In the case of my direct ancestor Maria Elizabeth, she was registered as Jones in 1839, which is her mother's maiden name, although she was baptized as Hughes and named as Hughes on all of the censuses from 1841. I can only find her brother Charles Henry on the GRO index, who was also registered as Jones.
I cannot find the other children on the index, Henry because he was born in 1836 (I have his baptism, and he was baptized Henry Hughes) and I think they would all have been registered as Jones, as they definitely never married. I'm still waiting for someone to come up with an explanation for why a middle-class, respectable family would have behaved this way.