There seems to be other things to consider here such as how this young woman will survive in supporting herself. I did not see any mention of job or college. I also didn't see any mention of friend's & how she is with them or any other relatives.
A lot of unknowns such as how your husband feels about all this. Having an older child at home means laying ground rules and consequences if rules are broken. Laying them down and drawing up a contract highlights the seriousness of this, plus seeking the cooperation in making it up gives your 21 yr. step-daughter empowerment in her choices.
I don't know what type of relationship she has with her mother beyond it sounding like there is a lack of respect that she would slip into verbally abusing her. Making it known that this is unacceptable is important. I am idealistic and look for opportunities when the door is open to explain reasons for the "why" it is wrong but try to stay away from "lectures" and the "I know, I know's" when you can certainly see the behavior doesn't match up...lol.
Relationships take time to build but some things have to be nipped in the bud...sounds like something was allowed to take hold in her home. But meanwhile, if it should happen you are approached or your husband, you will have thought this out and your seeking other views gives you more of an idea what to do.
Bottom line is how you feel and making it known to your husband so you are on the same page. If your marriage is fragile, would it withstand the possibilities that come with your step-daughter? Does she have a good relationship with her Dad? Will she listen to him? Will you have time to build a deeper relationship with her? Do you want to?
I think your being honest with yourself and your husband is very important. If your husband has been verbally abusive and called you names, one can understand somewhat where this example came from. If your husband is still trying to break free from this type of behavior, will having his daughter around, make it worse or will he want all the more to set things right? Lots to consider.
Best wishes in the outcome :-)
It seems you already have the problem of verbal abuse. Like father like daughter? Her appearance will likely reinforce his abuse. Are you sure you are with the right partner?
Do you belong to a culture where it's normal for a 21 year old woman to live with her parents, instead of have a place of her own?
This girl has gotten away with it at home, and if and when she comes to your home it is just a matter of time before you are the recipient of her bad behavior. I would not do it for all the reasons you listed. Any progress you made in your relationship will quickly fade into the background. And where does she go once she starts this in your home? Mom needs to let her daughter know that she is of age and if she cannot live by moms rules, it may be time to get a job and an apartment. This is not a child and needs to be learning some responsibility as well as taking the consequences of ones behavior.
I agree with Margy. Based on her current behavior, I wouldn't allow her to move in... you might have a difficult time getting her to move out if the arrangement doesn't work. Besides, you and your husband have your own issues to deal with... her presence along with her problems could complicate matters.
Your step daughter is an adult and should be abusing noone , do you know what form the abuse takes or is this the word of the other parents , perhaps speaking to the daughter would be good is there any reason she would behave like this . I dont think it is too good an idea at the moment to take her in ,if you have had problems of abuse , is this a family behavior/ is her father that is also abusive ,could she have learned this from him.The dynamics here sound problamatical I would think hard before you make this move .you may be sorry, perhaps some help for the' child' would be good but remember she is an adult and has to look after herself.