You need to be very careful!!!  This is not just a step issue as it could easily morph into a legal one.  Having a marriage fail is painful enought - being accused of sexual abuse could ruin your life, career, extended family, and financial security.

If you are still in the house, I would push my spouse to articulate that they really do not believe these accusations.  Then, I would NEVER be alone in the room with your sd.  I would also suggest to my spouse that I need, for my safety, to have sd get her own place.  

Good luck.  I think you are going to need it.

This is a bad situation. I would suggest to you that since your sd is accusing you of being inappropriate with her, you bring up the topic of installing a camera in the home. If your dw truly believes her daughter then she should be all for the idea, as would your sd because it would prove her case. Of course it will prove that you haven’t done anything wrong and works in your favor. Suggest a camera. You can find a reasonably priced one on amazon.

You and your wife were having a heated discussion when the accusation came from her so that can be repaired if you want it to be. 

Your sd is 19 and accusing you of touching her. She needs to find another place to live or you do. Never stay someplace when these kinds of accusations are being made. You need to protect yourself.

I was also going to suggest the camera.  I think you need that, at a minimum, to protect yourself from these allegations.  However, the bigger problem is that the lying SD, who is making serious accusations that could land you in jail, is living in your home (or at least it sounds that way).  IMO, one of you has to go.  Either she moves out, or you do.  It is not worth the trouble that can result to be living under the same roof with that evil girl.  

What is the financial situation in your home?  Is everyone pulling his/her weight?  

Set up nanny cameras and a digital tape recorder in your shirt pocket.  Then tell her a few things straight out that will be recorded.  You have a right to protect yourself.  

Take it to a Victim Advocate, usually located in your local Law Enforcement office.  They usually do not charge $$$$.  They are very helpful.   I have had Victim Advocate help me, they are great.

If your wife wants proof, give her proof.  However, I would show the webcam footage "proof" in court as  you take her for every penny she has with the goal of leaving she and her lying POS waste of skin toxic spawn living under the local overpass.

Grrrrrrr!  This woman and her toxic spawn piss me off to no end and I am not the one married to them.

Find the best divorce attorney in town, form your exit strategy and go to war.  SD-19 is an adult and as such her lies are a crime and can also bear severe financial consequences.  I would make the destruction of these two toxic  people my sole life focus as you work your exit strategy.   Take care of you, move back to your former home and never again tolerate a toxic person in your life.

Good luck.

invest in a nanny cam. protect yourself.

The cameras are a very good idea.  Sadly, the young lady needs to go to counseling, along with the mother to solve this problem.  She is making claims and one never knows what any one will do these days so you want to protect yourself.

We all got hit between the eyes by stepkids when we engaged in our relationships with their parents, so please don't feel weak or foolish!

I like the camera idea.My guess is that your wife said she believes her daughter because she was angry in the heat of your argument and wanted to hurt you. However, SD could still accuse you to police without your wife endorsing her claim, so you must protect yourself.

If you're back on good terms with your wife, I'd gently ask her if she really believes SD's accusation. You need to know where she stands. But before you ask that question, I'd get that camera installed and make sure it's recording her answer. Or better yet - ask her by email so you have her response on the record.

If SD made this accusation a while ago, what has been done since to address it? Your wife can't possibly expect you to continue living with SD after such an ugly (and threatening) accusation. Time to talk about moving SD on. If your wife isn't open to getting her out, then I'm afraid you'll need to think about getting out.

In this day and age, you cannot afford to take chances with accusations about sexual inpropriety.

Great to hear that it worked for you.  Do not take the cams down.  Keep them recording.  Just in case.

Keeping taking care of you.  I hope the improvements in your marriage continue.