AITA For lashing out at my grandmother for telling my (half) brother who my biological father is, whilst unbeknownst to me?

I, 13F only recently have been told that my dad who I believed was my biological father is not my biological father, and my mom’s sides grandmother has used that fact to ridicule and discredit me now that she knows it upsets me.

I was told by my dad’s sides grandmother, who I believed was related through marriage on my dad’s side, but is actually related to me biologically. To not confuse names, I will call my biological father bio, and call my dad, dad.

I was visiting bio’s sides house since we don’t see each other often. She lives near enough an hour away.

For context, I live with my grandmother, father, and brother. My mom lives with her boyfriend.
When I came home from my visit after my mom drove me home, I told my mom’s sides grandmother about how I was told about my real father. He couldn’t be here because he was abusive. Not only to my mom, but to me as well.

My grandma would always make dodgy comments to me about how we have to wait and see if I turn “psycho” like him, (he was mental) or how much I don’t resemble my dad and how I look like bio. Even weeks after I was told.

My brother was away with my dad on vacation with the rest of the family, and while it was just me and my (mom’s sides) grandmother, it was lonely for me because I didn’t get comforted in the way I need.

When my brother came home alongside my dad, I sat down with my dad while my grandma was at work and told him how much I loved him and nothing changes between us two.
As soon as my grandmother came home from work, she felt the need to tell my brother, regardless of me telling her that he is delayed mentally, (she knows this), that he wouldn’t understand the concept, and all that he would take in is that we arent related. He is 9 years old and wouldn’t understand.

And she told him. She was in the kitchen thinking I couldn’t hear her, but I heard everything and I confronted her gently, and she got defensive.

The final straw was today, when she told me, “You have that same vacant smile (Bio Dad’s Name) does.”
I lashed out at her and I told her that the comments she had been making towards me were inappropriate. Telling me in happy moments between us two how much I resembled my bio father, assuming the worst of me and assuming i’m a psycho waiting to happen, and the nail in the coffin: straining the source of connection I had with my little brother.

I knew it would go in one ear and out the other for him, because the next day we were playing an online game together while I was on the phone with one of my friends, and I was teasing him and he said: “we arent related.”

TLDR: My grandma told my brother about that we don’t share the same birth father, regardless of the fact she knows he wont understand the concept, as to being so young and after making a dodgy comment weeks later, I yelled at her.