AITA for saying my cousin’s intellectual disability is probably because of the incest

Throw away account cause im mortified.

My(21F) parents had a get together lunch among close family members last sunday. Me and my cousins were adding wedding dates from the piling invites to the calenders to make sure we don’t miss any.

I was joking about never understanding the familial ties in our native language (as in how you call cousins, or aunts or just the word in our language) as I was used to learning those words in English. My grandma, through my mother, was joking about how I am being too western and was helping me connect how I was related to the people in the invites.

Four invites in, me on a roll with being too loose mouthed from all the food I was eating asked my dad how the father of the bride in the invite was related to him. He said cousin, and I should have stopped there. But did I? No.

I was bored so I asked him to detail it more. He said well the bride’s grandfather and his mother are siblings. He paused, and added a "Well I would also be his (bride’s father) uncle too".

Now listen, I know incest used to be a thing. I know it used to happen. I didn’t know it happened in mine. So I grimaced and started laughing uncomfortably.

My uncle started explaining more seeing my disgust. Long story short. My grandfather is my grandmother’s uncle. (My grandmother’s mother and my grandfather are half siblings (I editted it from step to half siblings. I made a mistake in the wording), so my grandfather’s dad and grandmother’s grandfather are the same person).

Me and my cousins starting making those gagging noises while pushing the invites away and grimacing while walking around and our parents tried overcompensating with excuses. I know they don’t owe explanation for the choices of their parents and the others preceding them. The generation above mine share the sentiment that incest is a big NO. I don’t know why they kept defending it but yeah.

Here is where I said something fucked. I asked my dad if he realises that his grandfather and his mother’s grandfather are the same person. I added it with a "You know children born from incest have higher chances of intellectual disabilities right?". My cousin added a "Explains why my brother has that". And I immediately started nodding and said exactly.

My cousin does have mental disability and is non verbal.

Both me and my cousin were screamed at and told to come back home later. I know it is fucked to say that but me and my cousin were just getting more and more uncomfortable. We came back later and apologised. But my uncle and aunt refuse to speak to me and my cousin.

So, AITA for saying that my cousin’s intellectual disability is probably because of the incest?

EDIT

Adding this from the confusion in comments. I sent this to my cousin and he asked me to add some more information.

1. They are blood related. I worded the post wrongly by using step siblings. My grandmother’s mother and grandfather are half siblings with the same father.

2. One main reason for the disgust was me and my cousin already knew that my grandmother was 18 when my grandfather was in his early 40s when they got married. The age gap with the added knowledge that they are related was one of the main reasons.

3. Another reason why I spoke on it was because last September we attended a wedding where the couple were second cousins and had blood relation so while my parents and their cousins have moved away from this, others in our families still commit incest. This was why I made the comment on intellectual disability from my parents defending it while incest is a thing in the larger family circle.

4. I understand that I was being an AH and so does my cousin.

UPDATE

My cousin and I since posting this have talked to his parents about the incident. We apologised for the comments and we are good.

I just have to address this, me and my cousin weren’t mocking our brother for the disability. My cousin’s parents were hurt because the way it all went down made it look like we were blaming them. Me and my cousin apologised and said that we were more hung up on what happened between our grandparents and my dad excusing it. We made it clear that we knew they were never at fault and apologised again. I had also mentioned about the wedding we attended recently being incestual and about intellectual disabilities which lead to my cousin making the comment about his brother.

I love my cousin and the cousin who made the comments definitely loves his brother. My uncle and his aunt know that. My cousin is the closest person to him. My cousin has also gotten in multiple fights with other family members over comments they have made about his brother over the years.

Both of us went too far with our comments and his parents understood that it spiralled at the heat of the moment.

My uncle at the end jokingly added that as a punishment me and my cousin should be attending all the weddings we got invites for because both of us have an habit of skipping them.

We are fine, but thanks for the comments.