What do you think he's doing which is bad? If you think he's a danger, you should warn your sister, it's for her own good, if you think he's dangerous or he's doing something bad, you should try to see what she thinks, if your sister tells you to stop, just stop.
I think he lie. he don't tell truth about himself. but I'm not sure. my sister is intelligent. I believe she'll have a good decision.