Thanks for your help and interest Gadget and Annie. The information links have been very interesting.
As suggested by Gadget I have compiled a chart of cousins (I can identify) who have James and Ann in their lineage.
They are all in the 3rd and 4th cousin range.
(Any 2nd cousin matches are from my grandfather's sister so have the same descendancy from James and Ann as I do.)
For more data I have also included my full sister's match information too.
Match info for those who only have a single line of descendent from James and Ann:-
Match 1 - shared cMs me = 71, my sister = 114, predicted relationship for me = 4th cousin and for my sister = 3rd cousin, actual relationship = 3rd cousin twice removed, James and Ann relationship = 2 x great grandparents
Match 2 - shared cMs me = 48, my sister = 55, predicted relationship for both of us = 4th cousin, actual relationship = 3rd cousin twice removed, James and Ann relationship = 2 x great grandparents
Match 3 - shared cMs me = 33, my sister = 54, pred. rel. for us both = 4th cousin, act. rel. = 4th cousin once removed, James and Ann rel. = 3 x great grandparents
Match 4 - shared cMs me = 51, my sister = 39, pred. rel. both = 4th cousin, act. rel. = 4th cousin once removed, James and Ann rel. = 3 x great grandparents
I think the predicated relationships are pretty right, so, in answer to my original question, the multiple lineages to James and Ann don't make for errors in predicting relationships.
Thank you for helping me work this out
I really appreciate it.
Marlene
PS I can list the 2nd cousin matches too if anyone is interested.