My ex-wife has remarried and my 14-year-old daughter lives with them. What are the halachot regarding yichud if she finds herself alone at home with her step-father?

According to R' Moshe Feinstein if they are alone for short periods during the day, there is no problem, but at night if your wife leaves she should make arrangements for her daughter too. She should not leave the step father and daughter for extended periods. I would add that if your daughter ever gets some uncomfortable vibes from her step father a much more strict approach should be considered.