Hi Lisa,
I am sorry to hear about the progress of your father's cancer. I understand your devastation & desire to have him here forever. Unfortunately, a part of life is death & it is pretty tough to see this progression.
I wonder if a part of the reason his second wife is being controlling is that this is a way she copes. The first thing I did (quite unconsciously) when I found out about my husband's cancer was try to put order & structure around the chaos. I have discovered that this is the way I cope when things are out of control. I also wonder if she struggles to talk about what is going on because it is too painful for her? She may not have the ability (either emotionally or otherwise) to give you the answers you are seeking.
Are you able to talk to her about the way you are feeling? Do you get any time with your dad on your own?
I have started seeing the palliative care counsellor, and while it was a distressing conversation - it has helped me to adjust my expectations and 'normalised' some things (like my husband's increasing grumpiness). It is ok not to cope all the time with this stuff. It is hard, and you do need to let the emotion out.
I wish you the best on your journey.
Von