I am living with a host family and my little brother (9) spoilers movies all the time, he told me that (careful SPOILER) spider man was gonna die in this one movie (forget the name) while we were watching it. And I general when we watch movies together as a family he likes to say stuff to the other family members (who all already saw the movie) doesnt this \*inserts spoiler\* happens next or doesnt the person XY dies in the car crash? 

&amp;#x200B;

Anyways, today we were watching star wars together and he said this movie is the worst movie ever and I said "Please dont spoiler anything" and then he kept talking and I said: "Seriously, please dont tell me what happens".

He said he wasnt gonna spoiler anything and that was it and I did not think of it as a problem.   
A few hours later his mom and my hostmother came upstairs and said she wanted to talk to me about something that happened. I said sure and she asks me about what happened downstair in the living room and that her son was quite upset and asked me if I was just joking or if I was being serious. I said I was being serious because I hadnt seen this movie yet and I wanted to watch it without knowing whats gonna happen. Then she said he is only 9 and he is just gonna spoiler stuff and theres nothing we could do about it. Then I added that I could apologize and she was like yes, please". I didnt feel like apologizing but since this is not my actual family I did not want to have an argument with them.

Maybe I was too harsh and I just do not see it? 

AITA?

(excuse any spelling or grammatical errors, I am not a native speaker)