[Merry]

I'm not an expert, but I think you need to consider where in Wales the family were from.

I had a friend, b 1938, who was born in Denbighshire (right at the north of Wales) and she didn't speak a word of English until she was sent to a boarding school in England when she was aged 8. Speaking only Welsh was normal for v North Wales at the time. Interestingly, to my ear, people from N Wales have far less Welsh accent than those from the south, but I should probably say they have a different Welsh accent which is less obvious to people from outside Wales!

In general the further down the country you came the fewer Welsh speakers there were. These days Welsh is taught in Welsh schools which means more people have some degree of knowledge of the language than a generation or two ago. There's a campaign to reach 1m Welsh speakers and the population is 3.25m. Of course this is a completely different thing to having no English as was the case in some regions not that long ago.