Is she any good? I'm sorry but was this not an invite to shag your sister-in-law? I apologise if this was not the case. As I'll probably be giving her sister one at the same time, does it really matter?
Live your life by the axiom of "Don't shit where you live".
So as long as you're not living with your sister in law, back scuttle her until her blood orange goes supernova. As has already been said if you're going to fuck up your marriage, make it worthwhile. I've always regretted splitting up with the first Mrs TMW over coming back late from the pub, when a few months previously I could have spaffed my wad up her quiet-as-a-mouse-but-fucking gorgeous younger sister, but didn't.
I did my sister it caused a huge row in my family, my mother didnât speak to me for years. I eventually spoke to my brother and I explained I didnât know at the time it would cause so much distress to the family. He explained that her funeral was not the most appropriate time to show my "love" for her.