I want to tell my sister to take care of her son
Why should you have to tell her that?
I care about her son and I don't want to see him get hurt by my sister.
I think that you should be careful with how you approach this issue. If you worry about your nephew, then I do think that it is okay to tell your sister to take better care of her children, but this is a sensitive topic for her.
I know. I want to do it while she comes back from work.
If you feel that it is necessary, then that could be for the best. However, I have to stress that you should be constructive in your criticism, and to offer help if possible.