Dream about older sister and friend in bed

Have been having a recurring dream (4th time)..about my older half-sister (16 yrs odler than me), having sex with a buddy of mine from gym. We have a good relationship..Shes in her 50s attractive but happily married, religious and wouldn't seem the type to do such a thing. My buddy has mentioned that shes attractive a few times,and complimented her pic on facebook, which she was flattered by so I don't know if that planted a seed in my subconscious. (he'd never cross the line in person) She gets complimented a lot on Fb by diff guys so perhaps there's underlying wandering eyes. I do know she was wild in her youth, as a nosy kid I found her diary etc lol. Growing up a few of my school friends had crushes on her. That may feed into this.

I should add, that I chat with a friend of sis, on Fb..and we have a lot of sexual type discussions, as she is the total opposite of my sis (very sexually open minded & upfront...into BDSM lifestyle ..sis has no idea). She gets me quite aroused often and sister has no idea we talk like that, so I'd wonder if that plays into the dreams also. The forbidden factor

The dreams all follow the same pattern- I'm witnessing it happen,very vividly like I'm "there" but not there..almost like watching a porn..when its finished she meets me in another room and is imploring me "you won't tell anyone right, i needed this etc" 4 times this type of dream, in recent months. My reaction in the dreams is one of arousal, but not "towards" my sister..more so just the taboo nature of the sex, the sounds etc