Daughter sabotaging relationship

I am in a three year relationship and now 8 mo engaged. We both had previous dysfunctional marriage of 18 years. We both have two children each boy and girl. However his daughter 21 years old is sabotaging our relationship every day. She brings chaos. She doesn't want to work or study goes out every weekend drinks do drugs and sleeps around. She acts savage is the wife and totally runs the show in his home now supposedly ours. She abuses him and manipulates herbdadvin every way. He is blinded and cannot see this and I love him to pieces. Our relationship is great as long as she doesn't exist. I cannot longer stand to see this abuse and manipulation. When I diplomatically bring the issue up he is defensive and I always end up in the wrong. She is so way off.. When I went trying wedding dresses fir our upcoming wedding I brought her and my daughter with me and she tried every wedding dress that I picked after I was done with them. Her relationship with her dad is not a normal one. Don't get me wrong he is a good man and respect her but she doesn't respect him at all she walks in to his room even when naked and her items in his room from brad to thongs even her vibratory in his night chest. I am so lost. Please help

The first thing I would do is avoid being trapped into thinking of her as competition. She is not, if she was or is then there is a problem with the man your going to marry. So she is not your competition. She is your family member that is having a tough time with the fact that she is not the center of her daddys world. I think just by the way you described her that your not coming from a place of understanding. She is obviously a young (although immature) young woman that needs a strong female role modle. Why dont you set some ground rules THAT EVERYONE HAS TO OBEY. No leaving personal toys out or all children need to be living on their own by 25. Then why dont you address the real issue. Assure her and him that no matter what happens in this world he will always be her dad and she will always be his daughter. If they need father daughter alone time together give them their space to do so. Because i suspect that them just knowing that nobody can or WILL try to come between them will be enough and them knowing you support that bond will do wonders for the dynamics of the relationship between you and her. It will also do wobders for getting an ear of the father and more support in addressing all the other issues such as drinking drugs exetra....i bet youll find out not getting daddys attention has been a problem long before you were around.

Yammo, Agree with Gwen (nice one, missus!), but just wanted to understand something: "she tried every wedding dress that I picked after I was done with them" Why did you have a problem with this? What did you think it meant or thought she was trying to say to you?