Your sister in law has the right to know but it isn't neccesarily your place to tell her. You should talk to her boyfriend and give him the chance to tell her himself if nothing happens of it then give her the papers, you never know people make stupid mistakes and maybe it was years ago and he didn't think it was important for whatever reason. Give him the chance to be honest, otherwise she might think that you are just trying to cause problems and won't listen to you anyway,