OK, so Jennifer Jenzen is actually Sarah, her assistant. Sarah reveals herself (to no one in particular) while she thinks Eddie is following her. By the time Sarah and Eddie come back from the big pipe-o-blood, Melissa is already gone, and she and Sarah are never alone before that for her to reveal herself to Melissa. Near the end, when Melissa's face appears in the darkness, she calls her Sarah. This could of course be excused because the house hears everything, including her confession, and Melissa is now part of the house. The real slip-up is a bit earlier, just before Sarah shoots Price. As she is walking towards where Price is hiding, she hears Melissa whimpering and is calling out to her. She says "It's me, Sarah." Why would she call herself Sarah, when Melissa thinks her name is Jennifer?