So I can see either of these happening:
The sister in law will realizes her love for the main girl and leave her brother for the main girl in the end. Happy Ending.
or
This will be a dreadful experience throughout, the sister in law at some point will use the main girl, she then will stay with the brother, and the main girl will have to move on. Maybe getting with that other girl. Bittersweet Ending.
I'd like to see the first one happen. Even though this is called My Unrequited Love, it doesn't mean that it won't become requited.