From Wikipedia:
Nest ferch Cadell was the daughter of Cadell ap Brochfael, a late 8th century King of Powys, the wife of Merfyn Frych, King of Gwynedd, and mother to Rhodri the Great, King of both Powys and Gwynedd.
On the death of her brother Cyngen ap Cadell in 855, authority over the Kingdom of Powys passed to her son Rhodri the Great who had previously inherited the Kingdom of Gwynedd on the death of his father in 844, thereby uniting the Kingdoms of Powys and Gwynedd.
It is unclear why the inheritance of Powys passed through Nest to her son, and not to one of the sons of Cyngen: Elisedd ap Cyngen, Ieuaf ap Cyngen, Aeddan ap Cyngen, and Gruffudd ap Cyngen. The texts of Welsh laws which survive to us were written down no earlier than the 12th century, but they provide no evidence that women were capable of transmitting legal title of kingship or lordship.
Equally, although the pedigree in a manuscript in Jesus College Oxford states Nest as the mother of Rhodri the Great, another pedigree in a fourteenth-century manuscript in the National Library of Wales records his mother as Essyllt ferch Cynan. There are no strong grounds to accept either manuscript as reliable, but it is reasonable to believe that the royal house of Gwynedd promoted the view that the Kingdom of Powys had passed to Rhodri the Great through his mother in order to legitimize their control over it. Either way, this possible genealogical manipulation became part of the accepted story of the unification of the two kingdoms.