sorter

I (18M) play video games on my XBOX in the living room. Today I was playing For Honour (an 18+ game about medieval fighting) with my mum and our dog on the sofa next to me. My sister (15F) was hanging out with her friend (~9F) from our block. They came in so sister’s friend could pet the dog, and i paused the game. When my mum asked me why i paused it (as i was in the middle of a fight) i said because its 18+. The friend then responded ‘its okay, ive watched 18+ before.’
I then responded, ‘i dont care, im not going to play this game infront a child.’ to which my sister called me ‘square’ and my mum called me ‘obtuse’ because sisters friend had said she had watched 18+ before so it was okay. They both chastised me for quite a while before leaving it, and i was left pretty confused and upset.
I thought it didnt matter because the friend was obviously younger than me and my sister, and could have been lying to seem cool, or she could have been telling the truth, but i still dont want to feel responsible for exposing a kid to violence like that.
If she had been 14 or 15 i wouldnt have cared, but i definitely watched 18+’s i shouldnt have at 9 and regretted it.
I guess im wondering if theyre right and i was being a stubborn asshole for refusing to play the game in front of my sisters friend, or if im right in feeling uncomfortable for exposing a child to that.

[edit]
I know my sister was just trying to piss me off, and probably make her friend laugh, what upset me was my mum continually telling me i was being obtuse and it wasnt a big deal for a while after they left the room. She told me off a little bit and i genuinely didnt understand how what i did was wrong or a big deal.