John,
I can add a bit more to the other replies today.
The Oxford Family History Society records show a number of presumed siblings for Leah, all recorded with the surname FATHERS and children of John and Margaret baptised in Great Rollright: Ann in 1782; Mary in 1785; John in 1787; John [again] in 1789; Willliam in 1796; Elizabeth in 1799; Thomas in 1802; and Sarah in 1804.
I can find no trace of a marriage in Oxfordshire between John FATHERS/ FAIRBROTHERS and a Margaret. There is a burial record at Great Rollright for John FAIRBROTHERS on 28 September 1832, aged 71, who could well be Leah's father. There is a baptism record at Great Rollright for John FATHERS, son of George and Elizabeth on 7 March 1762, which could also be the same chap.
Looking through the Great Rollright records, I also spotted the baptism at Great Rollright on 24 February 1830 of Sarah FAIRBROTHERS, the illegitimate daughter of Leah, a servant.
I hope all this is helpful.