I think I can be of help. In fact my grandfather Bernard EVANS who was born in Glamorgan, Wales had a younger sister called Dwenwyn. My great-aunt Dwenwyn married Arnold J. (perhaps James) Hollinshead from Lancashire, England in Montana after which the EVANS family et al moved to San Francisco.
I know that Uncle Arnold (as we knew him) and Aunt Dwenwyn had one daughter: Edith Anne(Edie). Only much later did I learn that Cousin Edie had other sibling(s) who were estranged from the family. I think that you may be related to them.
If you would like more information about them please don't hesitate to contact me.