Never dip your pen in the company inkwell.
I don't understand people who screw around with someone who's married, or otherwise committed. What's the appeal? Competition? Lack of self-esteem? Worse yet, so many of them believe that the married person will leave his/her spouse to be with them...which is the height of delusion. If they'll cheat on their spouse, they'll cheat on you too.
Oh well. Sorry about your sister.
Hope she figures it out soon.