What is the purpose of his sister sending me this message?

You're a woman, right? The above two people haven't figured out their gender yet. Maybe he is not good, so his sister sympathizes with women and advises you to leave him. Now that she has done so, I suggest you take a good look at his situation and give up if it is not suitable, because his sister has advised you so much. If she doesn't like you and wants to separate you in this way, I suggest you contact her more and let her know you better.