My uncle with ALZ & Parkinsons, is still at home with my aunt. He cannot be left alone due to serious short term memory loss and instability. My aunt is full time caregiver and 3 of us show up PT to give her respite breaks. When we step in to cover her, he gets upset and insists he doesn't "need a babysitter." He is very angry (with her, not us) that he cannot have time alone, and rants about this for some time after she leaves. We know not to argue, but i'ts taking longer to cool his anger. The fact that he doesn’t think he has a problem, is a very big problem.
Dementia care requires creativity and preemptive strategy.
Your uncle probably needs meds for his anxiety/depression. What is your Aunt's sustainable plan for this situation? Surely she doesn't expect everyone else to orbit around them just because she doesn't want to deal with his anger? For example: she can *hire* someone to come weekly on the pretense they are cleaning the house, then she slips out for a few hours.
Sadly you cannot reason with someone with a broken brain, so best to not even try.
Some kind of calming medication may be helpful prior to your aunt leaving, but there are no guarantees.