How should I deal with Mom's memory issues?

My 83 year old mother has had increasing short term memory problems for 5 years and was diagnosed with MCI last year (she scored 26 on the test, 27-30 is normal). Other than memory problems she doesn't have other issues - at least so far. Her house, clothes and person are clean and kept to her normal high standards. She still makes her grocery list but sometimes needs help finding stuff in the store. She still cooks, not as much as before but still eats well - just switching to stuff like cottage cheese or sandwiches that don't require cooking. I talk to my mom on the phone a couple of times a day and spend 4-6 hours with her at a stretch a couple of times a week. My dad has heart disease and more advanced dementia and is quiet aggressive and verbally abusive to Mom. I try to get Mom out of the house more, but she won't leave my dad. When Dad has upset her, she has more problems. The doctor says the stress is making Mom's problems worse. Until recently her memory issues were sometimes a nuisance, but didn't cause any real problems. I'm not sure if the "problems" now are mine or hers.

Her brother (W) died last week on Friday. When I called Mom around 7:30a the Saturday morning to tell her about it, she told me she knew he had died and someone had called her the night before. When I spoke to her again in early afternoon, she told me her younger brother had called and told her brother W was dead. When I spoke to her again in early evening, she didn't remember her brother was dead or that anyone had spoken to her. Sunday morning she got into a big argument with Dad because he said her brother was dead and she told him he was out of his mind. She spent 6 hours at my house the Sunday during our weekly extended family dinner and brother W's death was discussed several times. When I called her later that night, she "W is dead?".

When we tell Mom she has forgotten something she will say that "her memory isn't that bad" although she did agree to stop driving last year when I asked and then pushed me to sell her car since she didn't need it anymore.

So my question is - when she tells me her memory isn't that bad, should I try to convince her it is and give her some examples? When she doesn't remember something and wants to know why someone didn't tell her sooner do I say we did and she just forgot (which usually leads to "my memory isn't that bad")? I feel like I'm almost being verbally abusive to make her face her memory loss, but when I need her to adopt some workarounds (like writing stuff down) or give up something (like driving) I also feel the need to honestly discuss it with her. Mom is still "here", her personality and reasoning seemingly intact - just sometimes lacking the information she has forgotten.

During our call this morning, I asked her to write down that her brother died Friday and the arrangements that had been made and tape it to the fridge. Once she had heard something enough times it seems to "stick" so I'm hoping that seeing the information in her own handwriting will help her retain it.