I put sister in quotes because she's TECHNICALLY Daddy's oldest daughter. She lives across the country and only seems to call when she needs money (which flowed freely out of Daddy's bank account until I got full POA).
Sadly, I'm not sure if this diagnosis is genuine or just another ploy to try to get money, but if it IS true, I'm not sure how to communicate this to Daddy.
The smallest things seem to upset him and confuse him. He's WAY more emotional than he was when I was growing up (he cries about EVERYTHING). I think this would just push him over the edge.
What's worse is that burying another child (baby boy committed suicide) would really take its toll on him.
...I have no idea what to do
Just because someone has cancer doesn't mean they won't be around for much longer. Both of my parents were diagnosed with cancer decades ago, and they are now in their mid-90's. It all depends on the type of cancer, what stage and how treatable.
My sig other's daughter [40] thinks her Dad is her own personal ATM machine. She will use all sorts of excuses, including the "cancer card" to ask for money because she doesn't know how to budget, and those $200 pair of jeans look so good on her :P
So don't be in the position of sharing information that isn't validated, and as Maggie writes, it's not your place to tell him that anyway. I'd stay out of it. If she does tell him and he asks you why you never mentioned it, assure him that you didn't want to upset him, which is the truth and the most important issue involved.
What kind does she have and what stage is it? With dementia, your dad will probably forget it anyway. My mom is like that. She has no short-term memory and has already forgotten that my brother had back surgery. I'd say protect your dad at all costs as far as this greedy sister is concerned.