IPX-412 is a 2019 film produced by IdeaPocket, starring Tsubasa Amami.

It’s primarily a POV story. We don't know the name of the main character. We know he lives a happy life with his wife, Kana. However, Kana is eight months pregnant, and the main character is feeling a bit sexually frustrated.

One day, Tsubasa, Kana's older sister, shows up at their door. She's back after a six-year absence and asks to stay with them.
It's been a long time since the protagonist last saw her, and --perhaps due to his sexual frustration-- he now finds her quite charming.

Having spent six years in a country where human relationships are less strict, Tsubasa doesn't hesitate to tease her brother-in-law. And since his body shows he's not indifferent, Tsubasa takes things further and further, eventually giving him a blowjob on the couch while her sister is in the nearby kitchen.

It could have ended there, but Tsubasa gets caught up in the game and doesn’t miss a chance to turn on the poor guy. The dinner scene is particularly well done: when he bends down to pick up his chopsticks, Tsubasa shows him just how wet she is. (Tsubasa Amami’s dress in this scene is incredibly hot.)

This sets the tone for the film: highly risky encounters where the sister is never far away. Even in the couple’s marital bed!

Several similar scenes follow. Then, one day, as it is a tradition in Japan, Kana returns to her parents' home to stay there during the birth and the first few months of the baby's life. Kana mentions that her sister is now alone in Japan, so they can keep each other company.

The protagonist is torn between his loyalty to his wife and his sexual attraction to Tsubasa. Tsubasa hasn’t left her room for several days, which heightens his sexual frustration, and he finally opens her bedroom door. He finds Tsubasa waiting for him, sitting on the bed.

For the next month, they follow the same pattern: letting sexual tension build up for three days, then releasing it by making love all night long.
In one of the scenes, there’s a very thrilling moment where Kana is on the phone with her sister while her husband is taking her from behind.

This is a very good film, where the husband’s hesitation and guilt are palpable, even as his frustrations take over.