Have you tried the Clover fork pins? They literally changed my life. My points are always lined up now because they force the nesting to stay, well, nested. I can even have less precise block sizes if I use them. Yes, $8.49 (US) seems expensive for 35 pins, but you don't end up using a ton of them as you're sewing things together.
I agree with the block-loc ruler for squaring HSTs. I also find with those the consistency in seam allowance is even more important. I put painters tape on my sewing machine to make the seam allowance consistent - I just keep the edge of the fabric against the edge of the tape. I also ordered these fork pins for this project, and they’ve been surprisingly helpful at keeping bulky nested seams exactly where I want them as I sew them together.
https://www.amazon.com/gp/product/B000PD0FRA/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Changing how your hand holds the fabric might help, like using a third hand clamp or embroidery hoop for sewing. Using fork pins for pattern-matching plaids will require less pinning, or switching to washable glue stick basting rather than pins. Either will actually hold better than pins, in my experience, it's amazing to see how much slippage regular straight pins allow.
I'd also consider consulting a physical therapist, there might be simple exercises to do.
Fork pins might work well for her. This kind is bent at the U, that makes it a lot easier to pick up and work with. Clover is my favorite brand for pins, needles and most any sewing notion. I'm not familiar with this brand but these pins have a plastic end that looks like a good option for your mom. I would not buy Dritz or Prym, their quality control is lacking, but if they have the best option you can find you could go through and toss the defective pins.
It might be best to give her a variety of items for holding layers together. Clothespins that open with a spring, those plastic coated clips, and large paper clips are all easy to open and hold and might work depending on what she is doing. (pulled these links to illustrate what I am suggesting only).
Another game-changing tool for helping get your corners to line up on blocks of all sizes: fork pins like these: https://www.amazon.com/Clover-240-Fork-Pins-Pack/dp/B000PD0FRA/ref=asc_df_B000PD0FRA/
Once you get the hang of them your life will be forever changed. I'm still not perfect at corner-matching but these pins combined with making sure to iron so my seams always nest, have dramatically improved my accuracy and decreased my frustration.