What should I do about this?

In my opinion, it is definitely not your problem. He has been LYING to you about this, which looks really suspicious in my mind. I'm not saying that he has been cheating or planning to cheat with her, but if it really is no big deal then he shouldn't be acting so secretive. It is wrong that he is trying to turn this around on you and make you look like the bad guy, when you are just trying to talk it out with him and find out the truth. I wouldn't plan to walk out just yet, but I would suggest maybe finding another counselor you can go to together. He needs to understand how much this is hurting and distressing you, and that it is NOT okay. You should be his #1 priority, and if his newly-reconnected relationship with this "sister-friend" of his is bothering you to the extent that it obviously is, he should have the decency to at least back off from her a little. If my husband started doing something like this, I'd be a wreck like you too. It is just not acceptable. Totally inappropriate for him to be acting this way.