Some of the things you mentioned can be checked and verified so that you don't keep jumping to incorrect conclusions. For example-
My mother remembered an aunt of mary janes called sarah and it was her fathers sister so that may be the sarah who married james hemphill but again its a fact I cant confirm ...
It's quite easy to look for the marriage of James Hemphill to see what Sarah's maiden name was- James Hemphill married (1876 Inishowen registration district) Sarah Anne
Peoples. One of the witness at Mary Jane Power's marriage was Peoples (
perhaps a connection of her aunt Sarah).