Dont tell them until you are ready for them to know, if they are overbearing now, be prepared for your whole body to be see as more "grandchild growing location", not yours. Seeing your medical information as family information, not private medical info.
Ok, you need to say no and accept they dont have rights over your home.
Dont cancel appointments again, you say to your dad if he says he's coming over "sorry, I've got an appointment. I'm free xxx/I'll pop into yours on my way back at yyyy time." If they come round unannounced, say to them at the door, "oh wish youd called first, I've got to go out."
(Theres a MNer who said her Nan used to answer the door with her coat on, if it was someone she wanted to see shed say she just got in, if she didn't want to see them, say she was literally just going out!)
What's important is you put a boundary in, your house is not an extension of the old family home. Similarly, you must ring the bell when you go to their house, message first and ask if it's ok for you to pop over, dont treat it as still your home.