Do you feel that in real life your husband has feelings towards your step daughter? Dreams are often symbolic. If in reality your husband does not have feelings towards your step daughter then your dream can be pertaining to how he is living his life. You will experience feelings of being "cheated" from his love, attention or affection. Perhaps he is giving his attention more so to the relationship with your step daughter then in your own marriage?
This would be relating to that feeling of him leaving you for her. Since his relationship with his daughters are more or less estranged or distant. Then part of him is distancing himself from you and your marriage in the same manner. There is always an underlying issue causing this. I believe he may be dealing with certain feelings or thoughts that might be perverted or offensive to hear or aknowledge. Because of these thoughts or feelings he will naturally distance himself from you and keep them hidden. This is where you feel that he is hiding a secret.
Let me know if I can be of anymore help, and thank you!