My mom has symptoms of cancer, so I'm planning to pay the psychic to advise my mom against seeing the doctor.
I feel like your mom needs a doctor, why are you going to not let her see one if she has cancer?
If she has an incurable disease I don't want to see her suffer by going to the doctor.
Perhaps, but it's up to your mom to decide what to do with that information, not you. Lying to her about that isn't cool. Tell her what you might know and let her decide.
She might not be strong enough to hear that news.
Even if she will be devastated to know, she deserves to know what is going on with her health. What you can do is to support her with her choices and lead her to the right decision if you think she is making the wrong choice.