The sister (age 21) of my sons friend from primary school (they're both 19 now) has just put a picture up on Instagram of a bruise on her arm. She put a big long post about the bruise not being deserved as she'd only looked inside a Christmas box of someone. I've just private messaged her and asked her who had done this and she said it was her brother. She had looked inside his Christmas box from his girlfriend and he'd told her not to be nosy and then hit her. I've advised her to speak to the police and she told me her Mum had also told her the same thing. She's said what's the point as they'll only probably give him a warning. I've told her that her Mums advice is correct and that if she doesn't it could happen again. I know her Mum won't push this. She's a very lovely women and her children are her world but I've always had a feeling that she has possibly learning difficulties or something similar. She's a single parent and had been for as long as I've known her which is over 15 years so no Dad on the scene to help her out. I don't want to wade in with my big size 7s but I'm so upset that this has happened and worry that it's not possibly the first time. What should I do?