First off welcome to PC. You'll find lots of support here. I'm sorry your husband had such a tough childhood. He was fortunate to end up with someone as wise and compassionate as you. Do you think it would be a good idea for you to have individual in conjunction with couples therapy? You both need to process this individually as well. I would wait until you've had therapy before you consider whether you want talk with the stepsister. I understand your shock as well as him not being able to share before now. This incident psychologically scarred him. I wish you both the best. I'm here if you need to talk.