<human>: Can you give me the gist of the text in a nutshell?

So as the title states my sister has been seeing this guy for about 3 months and I honestly have no problem with it, i know she is her own person and she need to live and learn. But she just texted me a question asking, " Is it okay to have (Boyfriend) tell me that I can't hang out with other guys." So from experience I know that question is somewhat of a red flag when having your partner tell you that.But Im not necessarily asking for advise in terms of how to handle the matter from my point of view. I would like for Reddit to tell me how you guys would handle it if you were in her shoes. Because I understand that I can't tell her what to do but I can make suggestions in order to hopefully guide her in a path that prevents bad decisions from occurring. But I'm all ears guys and/ or girls.
<bot>: Sister's Boyfriend is telling her not to hang around with other guys and need opinions on how Reddit would handle it if you guys were in her shoes.