I'm sorry you are going through this. It must be frustrating.
Obviously, they are all trying to shield you or protect you from this "information" and it's doing the opposite of its intention. Adults have been known to do that to children.
(A friend of mine and sister were prevented from going to her brother's funeral (all under age 13) because her parents didn't want her to go through it. Messed her up for years to come. )
Here's what I would ask of the person who probably knows:
Is there any health issue(s) that I should be aware of that could have been inherited?
Why was this information kept from me all these years?
How is it that everyone knows, except me? Why are other's feelings more important than mine?
Who would be affected if I have this information?
Is this an issue so large that others would be hurt if I knew?
I am going to counseling. Will you follow the advice and guidance of a professional counselor along with me?
Try to approach this calmly. You need to bring in an advocate (counselor or other family member) in with you at this time. Good luck.