the 1699 Will of Marjery Taggart als Bridson + annexed documents spell out the situation -

[some loss at bottom of page] unnamed son's wife;;dau Ann(not yet at years of discretion -all her cloathes), Jane;son-i-law James Tubman;bro Wm Bridson(+ son Wm);names Jony Taggart, Jane Samsbury;gch Wm Taggart;husb exor; 1703: Wm Taggart heir acks paid all due by death of mother;1706: Wm Quine (h/o Ann) acks paid from father Wm Taggart except that the heir is obliged to pay her towards her marriage;
1712: Wm Taggart states paid all due by his wife's will;
1713: decree signed by Bishop Tho Wilson noting that long, tedious + costly dispute between supvs of Wm Taggart junr and Isabel Harrison mother of ye sd Heir left to deceased Wm + succeeding heirs by his mother's will; - Wilson states that perishable items such as crop + team descend only to first heir + ye sd Wm Taggarts children have an equal share in husbandry gears etc

Looks as though Wm eldest son is dead by 1714 - his son Wm is the heir thus the Wm snr + junr in the will of his stepmother are gfa + gson;