Follow your heart, if you think he's a danger let your sister know, if you think he's safe, tell her aswell. I'd trust you if I were your sister. I just worry if you're ok.
I won't tell her anymore. my sister are worried when I talk about her boyfriend. today, her boyfriend send message to me again. he say "why you hate me girl?" if you're her boyfriend what do you do if her sister hate you?