My friend wants to be with his is ex’s twin sister

I have guy friend and he broke up with girlfriend who happens to be a fraternal twin. They broke up but still kept in touch with her family and got close to her sister. They have been on long calls and he’s telling me that he likes her because they have more in common and connect better. I told him to stop because well that’s his exs sister but he says no, he connects with her very well. What do you guys think of this so I can knock some sense into him.