I'm going to bang my ex's sister
Why would you sleep with your ex's sister?
I find her slightly better looking.
Going around having sex with people like your ex's sister is irresponsible. Choose your sexual partners carefully and only once you have developed a strong relationship with them.
But I still want to try it
That doesn't really mean that you should sleep around, how is your ex going to feel that her sister was taken advantage of? Don't you think it's best for you to finally detach from them and move on?