Thank you Jimmy,
I've been tracing the family and Emma and John were brother and sister (Charles Mullineux and his wife had a number of children). It seems that John Mullineux died in 1884. Then in 1890 Mary Ann Lucas Mullineux remarried to Joseph Molloy. So, William Charles Mullineux Morris b. 1879 and Mary Ann Lucas are in fact cousins and not brother and sister as I had thought. A tangled web. But thank you and Gardener for finding your way through the maze.
Kind regards
Robyn