HELP, my 8 year old sister hurt my baby

[Beastlybeautiful]

I was cleaning the bathroom and told my sister to keep an eye on my 6 month daughter, she's 8 years old. When I kept hearing my daughter going quite and then screaming and brbe going quite. I found this very abnormal as my daughters Very very calm and happy baby. So I creeped up on them without my sister realising and saw her pinching my daughter putting the dummy in her mouth to quite her up and then pinching her again to make her cry. I'm so confused because she's 8 so she has enough sense to know it'll hurt the baby and she could see the tears in my daughters eyes. I tried explaining to my parents but they brushed it off as a mistake and told her not to do it again. I asked her the reason and she said she doesn't know and that she isn't jealous or anything when i asked if it's that. I just can't imagine intentionally hurting a baby. My husband thinks she's got something wrong with her since my parents got divorced a year ago. Help

Maybe in future don’t leave an eight year old in charge of a baby. She’s just a child herself.
She probably only did it to see a reaction or get attention. I wouldn’t read too much into it.. but like I said maybe don’t leave them without adult supervision.

Hi get where you're coming from, but she was in a baby swing playing, and I was cleaning the en-suite while she was in the bedroom so I wasn't far at all

Never trust them together again. She is 8 and is probably very jealous but is old enough to know she hurt the baby and that it's wrong to hurt others. As for "having something wrong with her" she is 8, her brain is far from developed and in her head the baby took her sister away from her. Presumably she is no longer the youngest in the family either so may feel her place was taken.