I've been keeping a family secret from a former best friend. When she told it to me, she said to keep it secret from her family.
Are you comfortable keeping her secret?
She has not treated me well for a while, but her secret is something she doesn't want to tell everyone
Even though the two of you aren't as close as you used to be, it sounds like you are being a good friend keeping her secrets. If this is something she isn't comfortable telling her family, you're right to support her.
Thanks. I'm glad I didn't tell her family.
I'm glad that you have kept it to yourself as well.  Regardless of how this friend chose to treat you, revealing her secret would still cause her a lot of problems, so you should keep it, no matter what.  There is no reason to try to get revenge.