I don’t think it’s unethical per se to use your points and take your mom’s cash, but it just depends on how you look at the points.
I’m in the situation that I generally far more in points than I can use, so if I have the opportunity to treat my family to some of the benefits of “free” travel, I usually do. It increases my enjoyment of the points as it lets me be more generous without regret
In your shoes, I’d probably tell my mom I would get the rental car with points and let her pick up a few meals or something if she really wanted to contribute beyond the timeshare. But my mom is also no longer working and on a fixed income, while I have a well paying career I enjoy and healthy savings, so it’s not a one size fits all answer.