I'm going to date my sister in laws best friend and ruin their friendship
Why would you ever do such a thing like this?
Yes. I want to break her heart
Interfering with the relationships of others is a very mean thing to do. Also dating someone just to ruin a relationship affects two people in a negative way, so you are essentially doing twice the bad here.
I don't care I'm just bored and curious.
To want to date your sister in laws best friend just to try and ruin their friendship is kind of low. If you a bored than it may be a better use of your time to try and develop a hobby instead.