How can I help Mom care for Dad?

My mother cares for my dad who is suffering from dementia as a result of small strokes and small blood vessel damage from diabetes. In addition, he he doesn't walk well anymore and needs constant help to get around. While she is doing a great job, I am concerned that she is frequently mad at him. They bicker constantly about things that don't matter, and she's always frustrated by things he needs help with. She also gets angry at having to repeat herself all day. Is there anything I can do to help her learn to be more patient with him and realize it's the dementia and not him? They recently moved close by to my family because they had lived so far away from everyone in the past 12 years. My siblings are all in other states. I have suggested that she find someone to stay with him every once in a while so she can volunteer or do something else, but she said it would hurt my dad's feelings. I'm not sure I can volunteer to do it because my family needs me. My husband is very concerned about them demanding all my time - not that they do it now but will over time as my dad's condition gets worse. Also, they are financially able to handle an outside caregiver for brief periods. I'm sure this is common so I thought you would have some good advice. Thanks.