The Isabel Fothergill who married Christopher Harker in 1760 (1 May) was actually the daughter of Robert,
not Philip, Fothergill. She was baptised 22 April 1736, in Kirkby Stephen (as was her husband, on 16 Feb. 1736). It is hardly surprising, then, that two of their sons were baptised Robert.
Confirmation can be found in the academic paper "Appleby Goal, Onion Flatt (Waitby) and Robert Fothergill (1693-1779)" by Blake Tyson, in the
Transactions of the Cumberland & Westmorland Antiquarian & Archaeological Society (2003) 3rd series, Vol. 3: 139-156. This wonderful paper mentions Fothergill's relationship to both Isabel and her husband, Christopher Harker, and some aspects of their life. Unfortunately, the best online link currently (1 June 2014) available seems to be
http://books.google.ca/books?id=5JZnAAAAMAAJ&focus=searchwithinvolume&q=Isabel+ .