AITAH for uncovering a long kept (50 years) family secret and breaking it to my family about who my mother's real mother is?

I moved back in with my family a few months ago due to financial difficulties. Mother, my mother's Aunt (Auntie) and my mother's spouse, we all live under the same roof. The first month was a challenge to get used to each other's habits but we managed to solve everything with open communication and patience.

Mom's Aunt and I get along really well, grew closer in the past 2 months. We share many interests, our personality is similar in many aspects. She has a passion for psycho thriller movies and novels. We startend to talk a lot about this topic and bonded over it eventually. She became more open with me about her past.

Last week we started to discuss about my life goals and I told her that soon I want to have children. She asked me about names and out of respect to my belated grandmother (her sister) I wanted to chose her name to be my daughter's name, I also explained it to her that I had it in plan to give her name for a few years because my grandma and I were extremely close. She went silent and wanted to change the topic. In response I started to talk about how I was searching for baby names and I, being a curious person by nature, decided to push my luck and asked her about the reason why she was acting strange and why she and Grandma grew apart.

Background: Auntie and Grandma had a rocky relationship for decades, even hated each other up until my grandmother's sickness. For a longer period of time, even my grandfather started to hate on Auntie for no reason. They were avoiding each other on family events as well. Ever since I was born it was the elephant in the room that we never adressed, every family member just knew that these 3 people couldn't stand each other. When I asked about it as a child, no one knew the real reason, not even my Mom. So this subject was never brought up up until last week.

Auntie sighed and said that although my Grandma took a secret to the grave, she's not going to and lived with a guilty conscience long enough.

Long story short: She told me that Grandma and she were in love with the same man (Grandfather) and Grandma was promised to him by their parents, it was an old practice back in those days and that was a family tradition. Auntie had to stay at home to take care of the house and parents, after years was she just promised to another man. She rebelled, hated her parents and ran away, started to work and lived alone. She cut contact with everyone. Grandfather reached out to her after years, had a son with Grandma at that point but was still in love with Auntie. Grandfather left my Grandma for her (but never divorced), they had a child (my mom) together. Auntie never wanted children, they separated and Grandpa decided to raise Mom (since age 2) with Grandma. Saying that I was shocked would be an understatement. She asked if I was angry with her or not. I told her no in response to that, and that Mom has the right to know and had an ongoing suspicion for years anyways because the blood combination just made no sense in her case.

She expressed how relieved she felt after the discussion and added that if I want to tell it to my Mother, it's up to me at this point and that she basically has no regrets, also she made peace with her past.

Today I had a heart to heart with my Mother. We have (had?) a great relationship. I told her that what I am going to tell her would turn everything upside down. I broke the news to her, understandably she was furious. After that things escalated. She accused me of lying and after Auntie confirmed that I said the truth, she went ballistic. She wants to take a paternity test and went no contact with us and kicked me out of the house for an uncertain time, probably until the results are back.

AITA?