Should I say something? I think he'd being used.

I believe that my SO's brother is being used by this girl that he liked. They still hang out even though their relationship never took off but what I found strange is she knew he's last name before they went on the first date. Yet she didn't bring up that they are related until after (I'm assuming) he asked to be a couple. Now, I don't think they are related at all but she said she asked her granny and she said that they are related BECAUSE OF THE LAST NAME! Anyway I thought that eventually they would just stop hanging out but she still calls me to see if he's picking her up for college and to go to work (she calls me because his phone broke) and before I didn't really care what was going on but she called me at 6am! I didnt even answer the call and so she texted me asking if he's home. I was so annoyed, It's exam week and I couldn't be bothered and didn't want to lose sleep over it. at 9 before my exam I asked my SO if he thinks she's using him, he said yes but it's his call on the situation. The brother is a bit stupid when it comes to girls and always seems to think the best of them even when he shouldn't. I think I'm going to talk to her but not sure if it's my place I just don't want to watch him be used. But i also do not want to talk to him about it because he gets defensive. If I do this I'd rather go to her. Any suggestions?

Btw. We are all friends but I don't really like her anymore.