@finnegan1
"THANK YOU, Thank you, thank you!"
"I know taking care of [Mom] is a lot - if you get frustrated and need to let off steam, call me any time to vent." (Then let them vent without interruption, criticism or suggestions until they are done, then calmly ask how to help and listen again.)
"If it gets to the point where this is too much, let me know so we can figure out what to do."

Then:
If the person can talk on the phone, call them regularly and spend a little time - even if they repeat the same thing every time. Their caregiver can use that regular little break to take a breath or get a cup of coffee.
What can I do that will be the most helpful to you?
Can I come for a weekend and give you some time to get away?
Can I arrange to give you a break on a regular basis? (If you can't be there, YOU arrange a substitute caregiver and pay for them if needed.)
Can I take over managing the bills, ordering/organizing the meds, taking [Mom] to appointments or to the hairdresser or ?
Can I hire someone to clean or do laundry for you?
If you are not nearby, offer to find the local resources to help.

Yes, it might be a stretch for you financially to offer these services, but remember your family member is giving up life as you know it - their life is governed by the needs of the person they care for - no running out for coffee with a friend, spontaneous trips to the store, taking a weekend at the lake, watching their kids or grandkids baseball games.

Thank you for asking this question. I don't know how I would have managed caring for Mom (in her own place) if I wasn't able to share the duties with my sister and pull in my brothers as needed. And vent with my sister when it got to be too much.