Roz, hi
So sorry, maybe it's the cut and style of the clothes rather than the size, go back a generation and the cut was very different. Girdles, big knickers and vests rather and littles, bras were long line and rather drastic. Maybe find a catalogue and see if she fancies anything from Damart, etc.
Good luck with finding the right, comfy things that will work for you all.
Best wishes, Jo