How do you tell your adult step daugher how she is making you feel in your own home?

The fact that you have tolerated her behavior for so long is exactly the problem. She did not have boundaries and limits established when she was young and it will be difficult to do so now. The only thing you can do now is to set those boundaries and stick to them. It is entirely possible that it will drive a serious wedge between the two of you, but it is also your home and the safety of the children is paramount. Sit down as a family and set simple guidelines and rules for behavior. She needs to take care of her child and any children she charged with the care of as well as cleaning after herself. You will also need to get your husband involved. If he will not help you, then it can't work and you will just be the bad guy to both of them.