It is very interesting that you state that everything (almost) was perfect about this brother who came to your door. The one thing that made him not "perfect" in his facade was him encouraging you to lie to your husband about his purpose in being in your home.
You mention that everything about this brother was "perfect" in every way, from his physique to his book bag. You also mention how you, on a sexual level, were "taken" by him. You mentioned that you even brought in your mother to go with you.
It sounds to me like this is an all-encompassing thing. What I see here is your subconscious telling you that things are not always as they appear. By your dreaming this on a sexual level could be attributed to hormones, but I feel that it is more than that. I feel that by dreaming this on a sexual level made it very personal to you. Possibly even a life-altering thing for you. After all, what is more personal than letting go in a sexual manner?
I feel that by you dreaming that you brought your mother means that you had some trepidations and needed a level head with you to help you sort out your feelings.
There is a saying "Wolves in sheep's clothing". I think your dream was telling you exactly that!