It's not about the cost though, is it? It's about the perception of difference between the brands.
I'm assuming MiL is a similar age to my parents, and with that generation (and the one before them), there's a perception that M&S is quality (we all know that's not true, and quite a lot of it is shite now), so for the same reason I had M&S school uniform and Clarks school shoes courtesy of my grandmother (because these were quality brands back in the 80s), I'm guessing the MiL sees M&S as being a quality brand to gift.
Now Primark - which has some lovely, quality things now comparatively - was one of the original fast fashion, cheap brands, so it most likely has the perception to MiL of being a cheap, lesser quality brand (again, not necessarily true, especially with their increase in prices). Thus, it's a bit of a slight for DiL to be the only one to receive a different brand, especially one that has the perception of being cheaper and poorer quality (again, not saying that's true).
Or - to put it another way - would posters still be so quick to say there was nothing in it if it was the DiLs parents who had brought her designer pjs (don't know any designer brands!), but her husband Matalan or similar??