I don't know what to do. I want to start a relationship with my wife's sister.
Why do you want a relationship with your wife's sister?
Because she's prettier and nicer than my wife.
Even if she is prettier and nicer than your wife, you shouldn't want a relationship with her. It will ruin their relationship and it might break up their family when people take sides.
I don't really think that it will ruin their relationship though, since I'm going to date her secretly. I can't see someone who is that hot and just let her go. Also I would like to have her children.
I really think you're making a mistake. Dating your sister in law is wrong even if it's a secret. The secret will inevitably come out, and it could ruin the family.