I d/l and looked at the pattern. It will work out to about 6 1/2" around in stockinette. It's not specified that I can see but I'd think the gauge would be in stockinette. I measured a pair of socks I
that I worked in stockinette with fingering weight yarn and they're about 7" in circumference. These will be really nice when you get them done and stretchy enough that even though they seem small they'll fit your MIL. I think my worsted weight socks took about that many stitches but it's been a while and I didn't keep notes.
I think I'll stick with toe-up no swatch Lifestyle socks. Those don't look easy peasy to me. You're much braver than I!