You don't have to follow the rules, you know. You can certainly tell your brother if you think he'd be interested, and even talk to your grandmother. I bet it still hurts, sometimes, that she was made to feel ashamed (and my goodness, that far back, who knows which one of them had the no-baby issues???) about it, and *maybe* it would make her feel better to have someone tell her how GREAT it was that they adopted, and that there's nothing to be ashamed of.
The main secret I wonder about with my family is if I'm actually from one of my dad's brothers and my mom's sister.

I look SO much more like them than I do my mom and dad (though my dad, his brother, and I all were identical as toddlers). And it would kind of make me giggle. They all knew each other in high school, after all. But...I really doubt it logically.
I have three half-sibs from my dad's 2nd marriage, though they are not secret at all, and I'm glad to have them! I love thinking about genetics, and it's fun to see the traits passed down from our dad and how they changed with a different mom.