Was she right to tell me this

This girl knows her brother and I are friends so she text me today telling me to stop talking to him. Does she have the right to do that. I texted him asking if he likes talking to me and he said he does Bc if he didn't then he wouldn't wanna talk to me in the halls or at lunch.

I personally don't think she has the right to do that Bc if I'm friends with her brother and she doesn't like me then that's none of her business. You can't tell someone to not have a relationship with someone just because you don't like them.

Is she isolating me? I feel like she is.

Even when I text him about it he told me he likes talking but he needs to respect his sister.