You are absolutely correct.John I was married to Gertrude Bowman prior to leaving Virginia with Rhoda Fassit.Their son Sebastian I is likely the second born son of five children by Gertrude.The sequence of their births has not been determined, however, Nathaniel I was the oldest son and Edmund Bowman Cropper was the youngest son.Gertrude also had two daughters, Sarah and Elizabeth.The fact that Grandpa Edmund Bowman mentions all five children in his will of 1691 indicates that all these children were Gertrude's.
After sheriff Fassit died, his widow Rhoda and John I evidently carried on an affair which would result in the birth of two more sons, John II (born 1675 in Virginia) and Ebenezer I (b.c.1678 in Maryland).Sheriff Fassitt was first married to Tabitha Franklin, sister of John Franklin, whom Rhoda married after the death of John Cropper I.Rhoda had two sons by Fassit and when Rhoda joined in marriage with Franklin the house was likely loaded with children as Franklin also had children of his own. Both of Rhoda's sons by Fassit married daughters of Franklin from a previous marriage.
Obviously, Sebastian I was none too pleased when the affair of his father and Rhoda was exposed and then severely disappointed when he was basically left out of any inheritance from his father John I.
If you want to steer me in the direction you're trying to follow I might be able to be more specific with regards to the line(s)you are researching.