My brother is dating your cousin
If you are thinking of dating then you have to consider this: If your father had your brother with another woman then he has your father's genes in him and YOU SHOULD NOT be dating. What's wrong with this situation is that 1) she's a child and 2) you're 4 years older than her.
If your father (or mother) married someone else and your step-brother was the product of the former marriage then you are not genetically attached.
Because if you will date the friend will make your brother talk about you.
Wiki Answers is a question and answer site, and should only be used as that and not a dating service.
Answer: It's only wrong if you are in a monogamous relationship.
It is wrong if you have a crush or want to date his cousin.
There is nothing wrong with just talking, but if it is confidential information then regarding your personal life then I would be sure his cousin is trust worthy and wont go back and tell him about your life Absent any other twists...