I am mortified to be writing this
I can see why you would be mortified.
How desperate are you that you feel compelled to even consider taking advantage of your nephew who you should think of as a son?
Not only is this morally reprehensible but if your family did find out (because teenagers love to brag),you would lose the respect of your daughters and your sister would disown you.Not to mention it is wrong on all levels.
What kind of a role model are you for your daughters if you are willing to have sex with an underage relative?
You are 31 yr.old and you have yet to learn restraint?
Do you jump into bed with every guy who shows a sexual interest in you?
If you were my sister I would kick your butt and then press charges and then see to it that CPS was called.
Shame on you!