I'm so sorry you have to feel this pain. I know it well. My younger brother and his wife have been neglectful at best, and hateful at worst, to my parents over the past few decades, especially during the last decade when our parents lived with me.
Both my brothers were less than helpful during the decade our parents lived in my home. My resentment built, especially after Dad could no longer drive (due to dementia & pin strokes -- and Mom never drove), and my brothers both refused to help, saying it was my problem, since they lived with me.
My Mom died of cancer in June of this year, after an 8-month battle. My Dad has suffered from dementia for years, and had several complete breakdowns after she died. Last month, we had to put Dad in a nursing home. Despite the dementia diagnosis, my younger brother refuses to accept that our Dad is going to say strange and/or delusional things; he takes it personally, gets angry, and leaves. He is refusing to see our Dad this Christmas because of some delusional things Dad said. How sad.
My older brother had a complete turn-around during the last few weeks of our Mom's battle with cancer. He was there with me through her death, helping me with everything. He wants to be there regarding Dad now, but only for me, not for Dad (due to long-term emotional baggage).
The best advice I ever received about all of this was from a best friend who works for a care-giving company, and sees all kinds of family problems. She told me that I cannot control what Dad says in his dementia; and I can't control how my brothers react; but I CAN control my response to all of it ... that the best thing I can do is NOT let any of it change who I am. For instance, no matter what Dad says, I can't let it change how I love him and care for him. And no matter how hurtful my brother(s) actions are, I cannot let it change who I am -- how I treat them or their children, how I always pray for them, or how I view life in general. I cannot let their mindsets or behaviors change who I am. Best advice ever. I hope it helps you.
Good luck. My prayers are with you.