Hi Hangin In, and hello to everyone here. I've not been in a while.
Hangin In, I experienced similar with my Mom & Dad 10 years ago. Mom did chemo. Dad did not. They were both diagnosed with a rare aggressive cancer, at late stage, at the same time. It was bizarre for them and all of us.
Personally, if we had it to do over, especially since you said your Dad seems to have accepted this, I wish my Mom had not put herself through chemo. My father had a more peaceful experience than my Mom, except for his worrying about my Mom despite being in pain himself.
If your Dad is at peace with this, are you at peace with him choosing not to go through chemo? Because if he is okay with things as is... I would think that is wise. We experienced a wonderful grace in acceptance (not religious here so this is hard to explain) and it made our day to day life more meaningful. For all involved.
I have siblings as well and similar to your experience... there were things I didn't understand at the time and made excuses for them. In the last 2 years or a bit more I've seen things clearer.
You are one person and this is a lot for both you and your Dad. Enjoy your time together. Ignore your siblings and expecting them to help. They know the situation and if they want to be there, they can but you can't waste time on them. Focus on your Dad and you and getting both of you through this time. I can tell you this - the other side of things, I am the only one of all of the kids my parents had who is at peace and found the time caring for my parents to be a blessing and healing in many ways. I've not had any residual anger or difficulty accepting things. So if that's any help... just focus on you and your Dad.
Do you have in home help - palliative team? home care support? That will help a lot and is important I think.
I'll keep checking in regularly here with the other great people here so you have support here too. I'm thinking of you. Sending lots of good thoughts to both you and your Dad.
Carlyn