My sister has three daughters, aged 12, 7 and 2. The 7 year old has autism and her 12 year old sister is absolutely bullying her, in my opinion. Some incidents I've seen recently:
12 yo gave 7 yo one of her hundreds of teddies in front of her mum so she would be praised for being kind. 7yo was ecstatic and took it everywhere and played with it loads, only for 12 yo to demand it back a few days later and just chuck it into the pile in her room
7 yo said she was desperate for the loo on the way back from the park, 12 yo raced in to get to the toilet first and purposely sat on it for ages so 7 yo was crying and almost wet herself
7 yo loves drawing and spent over an hour on a picture. She went to the toilet and 12 yo decided the picture was in 'her space' at the table and shouted "if you don't come and move it in three seconds I'm going to rip it up" knowing full well she couldn't get there in time and would be hysterical at the thought of her picture being destroyed. I'm pretty sure she would have ripped it up if I hadn't intervened
7 yo carries a collection of belongings in a bag at all times that she likes to check and count as a comfort thing. 12 yo constantly threatens to, or indeed does, take the bag or some of the items from it and hides them causing much distress
12 yo constantly sets up 'competitions' between 7 yo and 2 yo, makes 7 yo think she's going to win and then makes her lose so she's upset
7 yo had a hospital appointment and 12 yo kept asking her why (knowing full well it was autism-related) and when 7 yo said it was just a check up to see how she's doing at school, 12 yo kept saying "it's because you've got AUTISM, AUTISM, AUTISM" repeatedly until 7 yo was crying with her hands clamped over her ears
12 yo will heavy-breathe on or poke 7 yo repeatedly to cause her discomfort and provoke a reaction
I could go on. It's become a lot more noticeable recently as 12 yo has been out with friends more and 7 yo is remarkably happier when she is. I approached the subject with my sister but she thinks it's just how siblings are
She said she agrees 7 yo is much happier when 12 yo isn't there so she struggles to punish her as if she grounds her then she'll just be at home more and horrible to 7 yo even more as she'll blame her.
Tonight they were coming in my car and 12 yo old locked the door so 7 yo was standing in the rain and couldn't get in. I told her in no uncertain terms that I think her behaviour towards her sister is despicable and she looked shocked, but still kept shooting her evil looks all evening.
AIBU to approach the subject again? It's way more than normal sibling stuff right? My heart breaks seeing 7 yo so on edge and tormented.