To dream that you (or your mate) have a mistress refers to your subconscious desire to end your current relationship. You are trying to sabotage your relationship in some way. Alternatively, the dream indicates that you are feeling neglected in the relationship. Perhaps you feel that you are not measuring up to the expectations of others, especially to your mate.
To dream that you are a mistress indicates your desires for the finer things in life.
To see a mistress in your dream suggests that you or someone is being lured or tempted into some negative activity.