Edge of Madness
(2002)

In 1851, Annie (Caroline Dhavernas) shows up at a western fort, saying she killed her husband.  Mullen, the local lawman investigates.

Annie was an orphan, placed into an arranged marriage with Simon (Brendan Fehr).  Simon was brutal and abusive with her.  Simon's brother George (Corey Sevier) lives on the farm with them.  George is attracted to Annie.

George tries to stop Simon's abuse, but fails.  George kills Simon with an axe.   Annie is delirious from the abuse she has suffered, and beats Simon's corpse with a stone.  Annie and George have relations.  Annie thinks she killed Simon, and turns herself in at the fort.

Annie learns she is pregnant.  She says the baby was Simon's, so the child will not be illegitimate.

George marries the daughter of a neighboring farmer.  

Thanks Joseph C!