Bf little sister message my brother?

So my bf little sister is 13. I’ve been with my bf for over 3 years now. Both of our family never been introduced yet but we are planning to soon. So anyways my bf little sister is very close to me. She looks up to me and always comes to me for EVERYTHING. This Morning she shared that she added my little brother(17 years old) on Instagram because he came up on her recommendation. At first I was a little taken back by this. Then she shared that she messaged him and that there just talking. She said there just talking about similar interests.

I honestly wouldn’t mind but my bf little sister is going thru her “boy crazy” phase. In the past she was “dating” a 16 year old. They wernt officially dating but she likes to flirt and the boy actually flirted back. My brother knows better than to entertain anything especially since she’s younger. But I haven’t confronted my brother yet saying that I know that there talking. Like I said I know there talking innocently but I’m just afraid that she might try to something and cause drama. 

Idk should I just confront my brother letting him know that i know and to tell him to back off? I know my brother is only talking to her because she told him that hes my bf sister so he’s probably being nice. My brother is really sweet. But her talking to him really makes me uncomfortable since they never met and she’s very boy crazy.