My sister came to stay with us at the weekend with her 2 kids (15 and 6) and what i witnessed has made me worry about my nephew but I'm not sure if I'm being ridiculous or not.
My sister treats my nephew (15) and niece completely differently and i feel like it is damaging my nephew. She is generally quite short with nephew but there were a few specific incidents that stuck out. For instance, niece hit nephew (hard) around the head with a control, she went to do it again and nephew stuck his hand up to knock away the control and nice started crying. My sister came in and shouted a nephew, and when my DH (who witnessed what had happened) tried to explain, she said it didn't matter and nephew should know better than to upset his sister.
When she left the room my DH asked nephew why he didn't tell his mum what had happened and apparently he said there was no point as she wouldn't listen anyway.
Also, on Saturday night we arranged for all of us to do something, but sister said that she couldn't as Saturday was her and nieces time and they always spend the night doing something fun. I asked if nephew comes and she said no, its just for them and she stays at home.
There was also other things said by both nephew and niece that worried me as well. For example, nephew says he has to spend the evenings in his room on his own (or out with friends/doing something), and is only supposed to come down for dinner, as his mum likes to spend time with niece. He also says he can't watch tv downstairs as if he does his sister will cry and his mum will make him change over. Also, niece said that when they got home on Sunday, they would be visiting her dad. She then added it would only be her, her mum and her dad doing something. I asked about nephew and she said oh he never does nice things with us because he is always bad. I late spoke to him, and casually suggested that he doesn't do anything with his mum as he would prefer not to, but he says he is just never asked. Whenever i try to bring it up, sister gets really defensive saying i don't understand how difficult nephew can be, however, he whenever i do see him he is always impeccable well behaved and tends to look quite sad and defeated 
Now, there is a long backstory, but basically my sister had a really difficult time surrounding the conception and birth of nephew (who is now 15). Her ex partner (nephews dad) was abusive and both she and my nephew have no contact at all with his side of the family. She also had a difficult birth and subsequently struggled to bond with him, and according to my mum has always been quite distant/uninterested with him (i was in a different country at this point so wouldn't know). Niece on the other hand was the complete opposite (easy birth, love dad, even though they are now separated, bonded really well). I guess this might influence the way she treats them.
Anyway AIBU to be concerned? I spoke to my mum earlier who basically brushed it off, but i feel awful. What can i do??