The horror film I saw on a channel a few years back. Daughter is lied to about mom being deceived but actually mom is being stored in a room in the house because she looks acts like a creature. Dad and her step mom hide this. But as she returns to visit her dad’s house from school she hears noises and decides to investigate,then actually sees dad with her mom in a tub where he hides her, towards the end she sees mom and finds herself next to her where they battle and she throws her down the lake. That’s as most as I remember
Sounds a bit like Beneath (2007) http://www.imdb.com/title/tt0467167/ but the details are a bit off. It is actually her sister that investigates, although her daughter is very much a part of the plot, since the mother often visits her secretly (she was burned and hidden away. The daughter thinks she is a ghost or some weird creature).