I have the urge to date my friends cousin.
Why do you want to do that?
I'm attracted to her and I love my friend at the same time
You want to date two people at the same time who are related, which is pretty much your fantasy now. Dating them both seems like it won't end well to me, choose either one or the other. Family matters are sensitive in this case, someone will get hurt if you choose to go with both of them.
I just want to make sure that I am picking the right family member to date, is all.
This is gonna wind up playing out very poorly for you. When they find out you most likely will lose both of them. I would recommend sticking with who you have now and trying to make it work.