Sarah, just one more thought.

I was wrong saying that IF your bf finds another woman, there would be more exchanges with her on his FB. That may not be the case if he were to hide that relationship. What would happen, though, in this scenario, is the following: the number of women friends he is adding would GO DOWN. Since he has limited bandwidth, as we all do, if he starts spending more time on another relationship, he would not have as much time for his casual befriending of men and women en masse, so he would be adding FEWER women friends than is customary for him. In other words, the intelligence brought by your sister speaks in favor of concluding that your bf is telling you the truth. I hope this will alleviate some of the nagging concerns you cannot help having.