WaitingImpatient YANOTBU!!!
And you did not lie, you said you had no money to end to her, you did not. You needed the money for your own medical treatment.
I had secondary fertility, I had a LOT of very expensive treatment. (We ended up adopting adorable ds to join our adorable dd and are very happy BUT the treatment we had was what we wanted to had and we spent our money to do it).
It is no one else's business AT ALL.
Please do not alter your appointment unless you really wish to.
Tell your sister as others have advised how this is a violation of your privacy and you take a very dim view of it. you are not obliged to lend her money or tell her about personal medical details.
The only thing for her to do now is apologise profusely and promise not to do this again. Then she can follow that up with a big pile of not talking about it at all ever again unless you initiate the conversation!
For other relatives and well wishes/snoopers/concerned relatives etc just put on your poker face and say your sister snooped on you and revealed private medical information you do not wish to discuss.
If you really want to stop the tongues I would say "This has upset me so much I am considering not having the appointment which we were booked in for. Please do not talk about this again."
Then I would have the appointment, do what I wanted to do, spend what I wanted. I would lock all paperwork away and make sure if your sis comes to your house she is not left alone with any private material at all. How sad to have to do that but she has driven you to it.
Then, deep breath, I would forgive her, privately if you prefer, and move on.
She has broken your trust but she will only alter your plans if you let her.