For a spouse visa petition and dependents, it would be a problem that you were over 18 when your mother and stepfather married but to adjust as part of the fiancee visa process, it should not be a problem, I believe.  However, without knowing all the specifics, if you entered in K-2 status you are not eligible to adjust except through the initial petitioner, even if you are now married.  you'd have to depart the US and reenter in order to immigrate.  However, without again knowing whether you have been out of status or for how long, I cannot advise as to what to do without jeopardizing your current status.