She needs therapy. However, I'm pretty sure she's like my family and will say "There's nothing wrong with me". My family hated my boyfriend for no reason whatsoever. They have alcoholism and drug issues. It's been over a year since I have spoken to them. However, I suppose it's harder with the mother since the attachment is stronger. I don't have experience with that since I only dealt with older siblings since my parents have passed. He needs to speak to her firm and tell her how it is and if she can't act right then he will stop talking to her until she can. I think she's jealous because he's the only child and feels like you're taking him away. If she could be more of a pleasant person then she could have him and you as a daughter, but the way she's acting is making her have neither.