You might consider Sockwell's Relaxed Fit & Diabetic Friendly Socks. Depending on the sock, they are made of equal parts Merino Wool and Bamboo Rayon with Stretch Nylon and Spandex in slightly different percentages. The link is to the women's socks.
https://sockwellusa.com/collections/relaxed-fit-socks?uff_i640td_tags=Women
I have worn this brand of sock with a similar material combination in the knee-high compression version for many years. In combination with my waterproof mid hiking boots similar to these, my feet have always been dry and warm or cool as needed for the season.
I wear those boots for stability on cobblestones and in slippery situations. I've worn them for years and they have helped to prevent turned ankles and falls on rough terrain, which can apply to cities. I could blame my need for them on my current age of 78, but I had near misses at a much younger age.
My boots are the lightweight mesh ones. I would never get the heavy leather ones. There are many styles of Keens that might work for your mom. If you go to the "these" link you can read much more about them. I have never needed to wear more than one pair of the socks at a time and I've never had a blister or hot spot anywhere on my feet with that Sockwell Keens combo.
I'm recommending the footwear as well as the socks because I'm assuming that the Christmas markets you're going to will be in Germany. Based on my 3 years in Nürnberg, I'd expect it to be cold, wet and snowy, but who knows these days. ;-)
Have a great trip and if Nürnberg is on your itinerary, you're in for lots of treats including Nürnberger bratwurst, lebkuchen, gluhwein and my favorite schaüfele.