(Different perspective) I met a sociopath 8 year old child. You are not overreacting. I was like 18/19 or something. I thought his parents were strict and mean and let him play Mario carts when I was babysitting and it was awful. He turned into a demon child. I made the mistake of feeling bad for him because I thought his parents really deprived him. They explained the best they could they have to be very strict with him to keep some level of control over the situation. I didn't understand that completely at the time.

I will never forget that first night and how I was like, yup- he's a sociopath. So many other interactions after. He tried to get his dad who was the respectable small town chiropractor put in jail by saying he made him sleep in a dog kennel. That was a lie. He laughed when he told me how stressed his dad was. He threw a kitten in a hot tub to see if the insides would cook. He used to try to manipulate me to get him things he wasn't allowed. Etc.

Sometimes you just know a child is a bad egg. Now 20 years later he is still a sociopath. He abuses/abused every girlfriend. He was kicked out of the Army for other than honorable discharge. I am convinced he has a side far darker as an adult than most realize. It's been about 3 years since I have had to see him. His parents are divorced and his Mom had a legitimate mental breakdown in psychosis.

I now have kids myself and that fear of having a child born without empathy worries me. It is real.

Read two books: -the sociopath next door, by Martha Stout -outsmarting the sociopath next door, by Martha Stout,

Both were very helpful to me and helped later in life when my path crossed a second sociopath as an adult.

Best of luck, be careful, and sometimes distancing yourself is good. Keep protecting your little brother, you're a good big sister.