Generations of Insestual Twins & Water Tank

by Chris
(Bronx, New York, USA)

A film came out in the 2000's about a girl that goes home with her boyfriend to visit his mother and notices by photos displayed that twins run in the family, and that for generations twin brothers and sisters marry each other and give birth to twins.

We also find out that her boyfriend has a twin sister they keep locked in the attic because she can't have kids. The girlfriend finds her and she is bleeding from all the failed attempts to get her pregnant.

The boyfriend is ill and needs to sleep in some sort of water tank that rejuvinates him.

In the end, the boyfriend's twin sister escapes and kills her brother and they die together in the water tank.

Any ideas?