Now the reason why many act the way you propose, is because the programming of the world, covers up the built in morals, and ego justifies
whatever it wants.
Speaking of 'Ego' wouldn't you say the majority of relationships are terminated prematurely because of ego?
As an enlightened being, you would surely agree that infidelities and straying are part of the 'lust' programming. However, it seems to me, in this
day and age that minor indiscretions are a major cause for couples sever all ties whether it benefits them or not. Where is the forgiveness?
I would be aghast if my wife were to stray....yet I would not be without some partial blame due to ignorance? I would feel betrayed, yet
understanding that she may need something else?
This whole monogamy thing is all good in theory, yet the human ego needs fulfillment. I couldn't fathom myself throwing my wife to the curb,
breaking apart a family, dividing all assets, and breeding a general atmosphere of hate for a minor fling. I would most likely forgive her and move
on. This is just me and old fashioned thinking I guess.
Now if she bolted and left her family, that would be a different story...
Ugh, too many variables..
Marriage is more than just 'puppy love'. It is a partnership and a sort of unconditional love that extends beyond carnal urges.
Your BIL is most likely at that 7 year itch, and feeling cool because some hottie at the gym is flirting with him. It could be a phase, or could not.
Your sister will eventually find out. (or maybe not).
Is it really your place to get involved without solid evidence?
These are tough choices, but if your sister and fiance are a good couple and you genuinely like the guy, maybe let this ride...for a bit. Maybe just
hint that you know what's up, and let it sort out.
True adults can work it out.