I was also abused over 40 years ago. I just started therapy about 2 months ago and my doctor wanted me to tell somebody but when I said no, she was very understanding and told me that I would do that when the time was right for me.
I was too embarrased and ashamed to tell them face-to-face as well, especially having cosciously known for over 25 years. What I had to do to be able to tell the siblings that I told was send an E-mail to them. I got replies from both of them within hours. They have proved to be very suportive of me and also agreed with my decision not to tell anybody else.
Before telling them, the hardest thing I everdid was tell my doctor what had happened. The therapy will not be easy, but once you tell about what happened, it will be like you just took a 10 ton weight off your shoulders.
Good luck with everything.