Is it okay to date your ex boyfriends brother?
Is it wrong to date my brothers ex? No, it is not wrong, as long as you talk openly with your brother about it. Dont get me wrong, it was a long time ago that they were together, so you dont need his permission, but being upfront and open about it, is a sign of respect towards him and her.
Is it wrong to like your exs sister?
Its totally ethically and by moral; its definitely gonna heart your sister deeply and silently she may be not gonna tell you that its not ok she is feeling bad about you and her ex. But you should now need to decide that do you love your sister or not ? If you love her dont ever go for that!