With Keri's approval, is there any chance you could approach Peri and let her know how you feel about her sister and apologize to her for how you treated her years ago? This is a dicey one because if you contact her first, Peri could be upset that Keri did not tell her about your relationship but, if Keri tells her first, she may still be angry over never receiving an apology from you. Any chance that Peri has just moved on and is well over any hurt that you caused her?