Jenet's gloss is good. Cadw is quite an interesting word, with numerous connotations in different contexts ~ keep, preserve, save, conserve, guard, hold, detain, retain, observe... Cadw is observe when you keep a tradition, and is retain when you keep what you have, which is why the heritage body chose it for its name I guess.

I think in this context it's an imperative kind of form: "Preserve you always". But I have only sketchy Welsh, so would need to check with a more fluent speaker to be sure.

Of additional interest for me is to realise on a more careful review that my original supposition may be correct, in that the young woman may have been born in my local village. I had surmised this originally because I thought the village name unique in Wales, discarded it on seeing that she came from the Valleys near Merthyr, but now realise that may have only been the living of her father in her twenties. She may have moved there at some point after being born here. I shall investigate further.

And yes Max, I too thought it really poignant to realise she was dead so soon after arriving, one of the many casualties of the mass migrations of all those decades, particularly in the light of her prayer to pay back her mother "a hundred fold".