Sorry to take so long to get back to you. This is possibly the same family. I have Mary Ann Diffen born in 1845 and Margaret abt 1853. Both Mary Ann and Margaret still used Diffen. It is on their marriage certificates. I have a copy of the announcement of Mary Ann's marriage to William Richardson. She is my g.g. grandmother. Death certificates of Mary Ann shows father Robert and mother Rosanna. Margaret has "Robin" and Rosanna. Perhaps they travelled under Devine as I am yet to find any evidence of their arrival in Australia. I will do some more research on the info you have given me. Do you have a tree?
Please stay in touch.