How do we assist my mom's aide to pull up or down my mom's pants to either change her or put her on the commode?

Maybe a walker - without wheels - kept in the bathroom for Mom to lean on while the aide assists her?

Hire another aid that is capable of handling mom. No offense to the aid, however the objective is to provide MOM with care that is needed.

Use a commode over the toilet. If u get new, there should be a bucket and a splatter shield. The Commode has a bar across the back, remove it. You can put the lid and seat up or remove them since the Commode has both. Adjust the legs so the splatter shield goes into the bowl of the toilet but not in the water. This will give Mom arms to be able to hang onto and push herself up with. Also, with legs its stable.

IMO it isn't safe for any aide to support the full weight of an individual with one arm while providing care with the other, I don't care whether they are petite or not. If your mother is no longer able to bear her own weight for a sufficient amount of time to pull up her pants even with the help of grab bars then you need to either
work with PT in order to regain that skill,
accept that toileting is a thing of the past and changes of clothes and incontinence products take place in bed,
or invest in a toileting sling for the hoyer lift

I agree with Geaton. Ask the providing agency to send you out an aide who can handle Mom’s needs but make sure you tell them you are not displeased with this aide, just have concerns she is not strong enough to keep Mom safe.

A Sit-to-stand is not that much different from a Hoyer except that it’s not quite as complicated to use. It would not help Mom with her toileting needs. It can be bulky and hard to maneuver.

I would request an aid that can do the most basic aspects of her job. She is not qualified to help your mom and may in fact be a danger to her.