We all expected that Mary killed Jessica - no OMGs there at all. The more shocking part is that yes, this actually was Mary in the house. Some would say that’s not shocking, but I still think it is, because I can’t believe they had Mary’s storyline perfected back then in season 4. But on top of that, what Mary was doing is shocking too. She was trying to get Peter’s pills, to frame him for Jessica’s murder which she was about to commit. Peter looked guilty as hell in 5A for killing Jessica - I’m kicking myself for not thinking that he was being framed by the person that was snooping around in his house the episode before Jessica’s death. That is truly brilliant. I’m so happy right now with this answer. This is a good mystery.
Teresa Parker was introduced in the family business storyline, whether or not she really is Peter's sister is left ambiguous.