Married after being diagnosed with dementia. How do I make sure my father's children are involved in his care?

My father was diagnosed with dementia about 3 or 4 years ago. He was one to never believe in getting married. He has been in a relationship with his new wife for over 30 years with a child from the relationship. He also has 5 daughters from two previous relationships; the oldest 54 and the youngest 33, who happens to be their child together. Somewhere along the way, they are going to get married so quickly without the knowledge of the three eldest daughters raised outside of their household with their mother but with the three that were raised in the household with them. His wife is also terminally ill with sclerosis of the liver and may not out live him. I want to make sure that if something happens to her that all of his six kids and not just the three raised by them are the only ones aware of what happens or make the final decisions on what happens with him. I am thinking to get legal guardianship and/or power of attorney just in case something does happen with her. I've made numerous efforts to make sure that they know that nothing is to happen without our notice as far as my dad is concerned. I think she married him so to take control of whatever he has and it goes to whom she wants it to go. I'm not really worried about what he has but that once she is gone, that he is still taken care of properly. His money and her money are both put into an account with their daughters name on it but he is wearing pants three sizes too big, not eating properly, and has lost an extremely large amount of weight. I have resulted in taking large meals to his house because there are times that I ask him had he eaten and they both will say no and I have a problem with that because he does have medication. I'm not sure of her true circumstances but she has an entire family to care for her but my dad only has his 6 girls. I just need advice on how to move forward in making sure we are involved in his care and well being.