Same dream for over 6 months

I wonder if you can help me. I have dreamt of my fiance (who I have been with over 2 years)of going for my sister, last night (in my dreams) he actually kissed my sister and when I asked him of what he was doing he did not seem to care... I don't know what it means I on & off always drem of this. Please help I have tried all dreams books but none answer my reasons for this constant dream.