We are only as sick as our secrets...
I am sorry perpetuallysad...that is hard....

I had a similar situation in my life..right befor my Mom passed away she shared that I had a sister I never knew I had...She had an affair and gave the baby up to keep me...we all lived under the threat(darkness) of this secret....
I would wait a few days...and then ask her again,,explaining the need for you to know your genetic history..to be aware of any and all health issues you might face in your or your childrens life...It is you right to know...
It would be hard for her to keep this secret buried when facing issues relative to your or her grandchildrens future...
Again,,,I'm sorry...but this is not a problem without a solution...but one you might have to work for..
With Care,
Lenny