Hi I think the parents firstly should get him to the Doc and get an evaluation ,it is very hard for parents to accept their child May have some problem.Does he get attention has he siblings and how does he get on with them,Do his parents react with Tolerance to his behavior. Perhaps you need to be firm in asking them to seek help from a Proffessional, you are a relative and help with him you can do this.
My question is how do you get a parent to see what is going on with their child and get them to have the child evaluated??
You can't make a parent get a child evaluated unless you believe that the lack of evaluation/ treatment is physically/ emotionally dangerous for the child. At that point, your option is to call CPS to report the person. This is your sister, so my assumption is that you are not going to do that ( I would not either:))
If you are uncomfortable having him around because of his behavior, you have every right to refuse to be around him-- that's your call. I assume you don't want to do that either (and neither would I.)
Since you have already broached this subject with your sister, his parent, and she has indicated that she does not want to take your advice, I don't think you can do anything else-- you've already gone beyond where most would. If his behavior/ issues are severe enough, his school will raise this with his parents-- they will bring it to your sister's attention. If she fails to act in her son's interests after this type of intervention from school, then the school will call CPS-- they have to. If the school never makes this type of intervention, then your sister will probably not have to do anything to change her thoughts about her son.
Its a tough spot to be in, but its a parent's job to take care of their kids-- and we all must abide by parental decisions unless we are willing to overrule them-- and that's when the law must get involved.