The House Sitter (2015)

When Kyle Lawrence, his wife Sara, and their teen daughter Amy return early from their annual lake holiday, their housesitter Rebecca has no place to stay until her next job, so Sara lets her stay with them for a few days. Rebecca plays the perfect guest and housekeeping help, and even assists Sara with renovating secondhand furniture. But while the Lawrences were gone, Rebecca had actually gone through all their belongings, especially mementos. Amy, who never got over failing to save her kid sister from drowning after she fell through the frozen lake, mistrusts and resents the invasive "substitute sister" who manages to make her parents believe that she's been abusing pills. When self-absorbed Amy and her obsessive quest for dirt on the newcomer wear out her boyfriend Travis' patience, Rebecca seduces him. Rebecca (actually just another alias) will stop at nothing to either find her perfect family or gruesomely punish everyone for their "betrayal."