So the other day I (20M) went to my sister's (26F) house to celebrate her daughter's third birthday. My girlfriend (19F) wanted to come with me. She had never met my sister before and was excited to meet the rest of my family aside from my mum and dad. But I knew there would be a potential issue. My girlfriend has autism, and she likes to carry around a stuffed animal with her named Mandy. She's not intellectually disabled (I think if you gave us an IQ test she'd score higher than me, honestly) but she does have some quirks with which I try to be as accommodating as I can. She likes to carry Mandy around with her in public, and I've never had an issue with that.
But I told her that she shouldn't bring Mandy to my sister's house. I suspected that if my niece (who again, is a three year old girl) saw Mandy, she was going to think it's hers, want it, and then get upset when she's told she can't have it. I explained to my girlfriend how this could cause a potential issue, and I didn't want my niece crying on her birthday. She said she understood but that my sister should explain to my niece that she can't have it because it belongs to her (she's an only child and has limited experience with young kids, and I don't think she realises you can't really reason with a child of that age like that).
So I then texted my sister and tried to explain that my girlfriend was going to bring a stuffed animal with her, but that it wasn't a present for my niece and she wouldn't be allowed to play with it, and that the present we were getting for her was going to be a Barbie playset that was wrapped up. My sister was very understanding and non-judgemental about the whole thing and said she'd keep it in mind.
So on the day we arrive and when my niece sees my sister holding Mandy, she immediately wants it. My girlfriend says "Sorry, but no". She looks to her mum (my sister), presumably thinking she would give her a different answer, but she tries to redirect her to the present we brought for her. My niece has a meltdown. My girlfriend isn't good with kids or loud noises so she starts having a meltdown too, which then creates a bit of a feedback loop where they're setting each other off. Keep in mind that both my family and my brother-in-law's family are at the party and are watching this unfold. I take my girlfriend outside to calm her down a bit, to which she finally agrees to leave Mandy in the car for the rest of the day. We come back inside and my sister has finished calming down my niece. The rest of the day was more than a little bit awkward. My sister's father-in-law made a comment that infuriated me about how he didn't know there were going to be two toddlers at the party but I didn't say anything (in hindsight I regret not confronting him over this).
When we got home I told my girlfriend that I was mad at her for upsetting my niece, that I told her I knew this was going to happen if she brought Mandy with her, and that she embarrassed herself in front of my family. She said that I and my sister didn't do enough to stick up for her and that my niece needed to learn that other people's things aren't hers. Now she's giving me the cold shoulder and I'm not sure if I should have handled things differently. AITA?
EDIT: Clarified that the FIL is my sister's.