My Mother is in denial about Father's terminal illness. Any advice?

My Mother is POA and my father has stage 4 cancer and has done 3 rounds of chemo. Has lived with cancer for 2 years. He was in the hospital because of a related brain issue. The hospital had to discharge after a month and we found a care facility. He is going to do sub acute rehab. My Mother hopes to have him strong enough to try a new immunotherapy treatment recently FDA approved. My father can barely sit. Soils himself, can't feed himself, and is racked with pain from the cancer. We (siblings) have said perhaps it is time for Home Hospice. To keep him comfortable and ease his pain. My mother is in denial saying that she hopes that he gains his strength and will be able to do the treatment. But the siblings want Father to have some quality of life, however much is left. Mother is saying she is waiting on the Drs to tell what to do. The thing is they gave them two options to continue or stop. We even had a conference call with the Doctors who kindly took time out on a Sunday Morning to do so. She said she doesn't remember. I don't know what to do. She says she doesn't want to give up on Father, we don't want to but the siblings don't want him to suffer needlessly. Is there anything the siblings (all adults ) can do IF we fear Mother is putting Father needlessly in pain, thinking she is doing if for the best intentions?