Not to mention that one of the Nestas has 4 husbands.
The trouble starts with the Grahams. They first appear in the time of David I and their origins are completely unknown.
Well that wouldn't do, so a pedigree had to be devised. I don't know why they went for Ernulf de Hesdin as an ancestor, but I suppose he looked suitably important-but-obscure.
Presumably whoever did that thought Ernulf's wife was unknown, so he conjured up a nice Welsh girl.
Then later it would be noticed that Ernulf had a wife called Emmeline (she was the widow of a Lacy, not the daughter). So two different pedigree-mongers would come up with two different "solutions" - one makes Nesta the 2nd wife, the other tacks on a previous generation and makes her the mother.
But the Welsh, having no actual records for that era, are committed to the view that their old genealogies are 100% accurate oral traditions. So a lot of effort goes into combining them and reconciling their many contradictions.