Wanna marry my father's brother's wife's sister's daughter

Yes. You can. Because thre is no existing relation between you two.

Whatever type of relation you have does not come under the prohibited relationships.

Go for it ..... Also make sure both the parties sign an affidavit before marriage that it is a NO-DOWRY marriage which may help you in future !!

yes you can marry that girl as this relationship dnt comes under the prohibited degrees under section 5 clause iv of hindu marriage act ,and dnt forget to register your marriage as in some states the registration of marriage is compulsory.