She is a cheat, a liar and evil and I believe she is cheating with my son's step brother but both are as good as it gets when it comes to lying and covering up things.

Why are you involved in this? Your son is the one that needs to figure this out. If you intervene, you'll only lose your son.

So let him be in this relationship. If this girl is a cheat, it will come to light sooner or later. Then you offer moral and emotional support to your son.

There are lessons in life that we each need to learn on our own. Sometimes mom has to step away and let her adult children figure things out for themselves.

Give him a smash

I will agree, if you feel you need to tell him, then tell him, But really it is none of your business,

If you have to talk to someone talk to the step brother

How old are all 3 of them? You do realize that when you are a parent, there comes a day when you have to step back and say that you did your best and the rest of their lives are their business?