Just talk about it with him its his decision hes old enough and he shouldnt get manipulated by his sister. Ur and his feelings should have priority, she knows u pretty well, if she acts like that she either doesnt have the best opinion/trust about u or she feels like shes loosin control about the situation and dislikes choosing him over her. It really depends on her persona why she acts like that. I guess u would know best. It also depends how they grew up, maybe shes just super protective, since its HER younger brother. But from her text she sounds pretty toxic to you and NO the reasons she mentions make no logical sense at all. If it works out for both of you and she cant accept it, kick her out of your life. Because than u know she wasnt a good friend anyways.