I suggest you put her off as best as you can. Would it be possible to tell her you are there to visit with your father when she starts to lecture and that you can listen to her in an hour or tomorrow or whenever is the end of the visit?
There are a number of different tactics you can try, to turn a conversation around, give it a think and see if you can come up with anything- my brother has always had suggestions for me, I hope you have someone to consult, too.
I have had to try different techniques on my own mother over the years, and the not calling and not visiting thing is usually my last resort. I am pretty good at distracting people from their topic, probably because I am myself distracted.
Only you know your family and your situation, so my suggestions are just that, suggestions that might not work for you. I really hope you find some way to make it bearable. I am so sorry your father is having such a struggle with his health and I wish the best for you and yours.