Your Sister's Sister
Hannah (Rosemarie DeWitt) finally confesses to her sister Iris (Emily Blunt) that she slept with Jack (Mark Duplass) after a heavy night of drinking (and despite the fact that Hannah is a lesbian). Meanwhile, Jack has found out that Hannah had poked holes in the condom they used, and he fears he impregnated her. Iris confronts Jack over sleeping with her sister and all the truths come out, including Jack learning Iris is in love with him. Hannah confess she did poke holes in the condom because she was so desperate to become a mother. She never meant to sleep with Jack specifically and wouldn't have done so if she had known Iris was in love with him, something she didn't discover until after the fact.
Jack leaves the cabin thinking it is best to keep his distance. Meanwhile, Iris and Hannah slowly mend their relationship. Jack spends a week away from them and after seeing a pair of young boys that remind him of his deceased brother, he has another breakdown. He returns to the cabin and says though he is not in the best state at the moment, he is willing to be a parent (if she wants it) to Hannah's child if there is one, and that he is in love with Iris. The three of them forgive each other and plan to move on with their lives.
The final scene is the three of them waiting with Hannah's pregnancy test to finish. We do not learn if she is pregnant or not but either way, we know that the three of them will be okay.