I've seen many stories where a plot point will be that the male lead was/is in love with his older brother's girlfriend/fiancee/wife, but sadly it's usually unrequited love.

Are there any anime/manga where that person who is with the older brother realizes she loves the male lead/younger brother and they end up together?

The only one I can think of is Please Love the Useless me. It doesn’t center around the male protagonist’s unrequited love for his brother’s fiancé but it is a definitely important topic in the story and to his development both character wise and relationship wise with the female protagonist.