Dad diagnosed with stomach cancer. Do we tell Mom who's bipolar, paranoid and has dementia?

It's spread to liver & lymph nodes. They live with 2 sons ages 59 & 61. Tension and arguments fly all day. We offered to take the folks to live with us. I have been a caretaker for over 40 years now and Dad trusts me and wants to move up to our home but Mom won't. The other 2 sons have never had any relationship with their parents and just started to do the shopping and driving. The house is full of tension and arguments and I just don't want Dad's last days to be spent there. They are all self centered.I saw them 2 days this week and was told by one of the sons that I better leave earlier since I was making things worse?? Neither of them talked to me or asked anything. When I mentioned that hospice will be involved and bluntly asked if he knew what he was getting into, he just bluntly said yes. I was not included in anything except if Dad asked for my help. Feels like my hubby and I are being pushed aside. We don't want anything except to help Dad. Please help me to deal with this? Thanks so much! Lore