T.H.
Technically your 18 year old is an adult and needs to be treated as such - if she's in school and you are supporting her she needs to be actively working toward a degree or certificate so she can earn a living, if she's not in school she needs to be working a full time job and contributing to the household, and if she refuses to do either of these, in my opinion, it's time for her to move out - a household needs only 1 adult female to run it and as a wife and mother, that should be you. And yes, if you allow her to do nothing but sit around and disrespect you, you have lost control and need to get it back. How can you expect the younger children to give you respect if you don't demand it from her?