Could May and Ann have been sisters? There is that possibility and I will go over my records to see if I can connect the two ladies.
I dont think they will be sisters. In the 1851 census, they are in different households. May with her mother Mary, and Ann with her mother Isabella.
If we have identified the correct Ann Fimister, her father, John, had died in 1849. It could be that her child John McDonald was named after her father.
I would look for that Ann in the 1861 and 1871 census, and see if you can find any link between John and that Fimister family.
At least you have a name of both parents, which is quite often not the case with illegitimate children.
John does seem to have known her name, and to me that suggests some sort of contact, although it might have been minimal.