If you are close by then that would seem to be your answer - you will need to self isolate and test regularly, but moving in with them for the period would seem the safest and least disruptive way of helping them whilst limiting spread.
Also, if they can prepare their own food and you live nearby, you can drop groceries off on their porch. Or you can ask what they want and pick up take out and drop it off for them. Here we only have to quarantine for 10 days, so it's not that long to go w/o cleaning, etc. Not sure what other help they need.
If you aren't worried about getting COVID (if you are vaccinated) you can probably go over there with a mask on and do whatever needs to be done staying six feet away and then sanitize yourself after leaving.
I've used meal delivery services like UberEats, GrubHub, etc for my dad.He couldn't deal with the ordering system so I did the ordering for him. I've even ordered his meals to be delivered to him while I was in Europe. You set up an account and they are prepaid, including tip, and can have instructions for no-contact delivery. It worked great.
If you aren't worried about getting COVID (if you are vaccinated) you can probably go over there with a mask on and do whatever needs to be done staying six feet away and then sanitize yourself after leaving.
All the precautions are more likely to protect you.