I'm fairly certain that the wife of Evan Owen and the mother of David, John & Glyn was a Jane Jones, the daughter of a Cadwaladr and Jane Jones.
As mentioned, with Cadwaladr and Jane (and their family) in 1881 is an Owen Jones of the right age listed as a servant. It would be nice to think that this Owen is the right one and was Cadwaladr's nephew but because the name is so common it could just be a coincidence, and not a particularly unlikely one.
It could be him or Owen could still be Jane's cousin and be somewhere else in 1881 or he could actually be Evan's cousin through an as yet unknown relationship of their parents.
Nothing is clear unfortunately.