My Sister, My Love (2007)

Incestuous relationships between sisters and brothers. Mainly brothers being highly obsessed with the sister while the sister is in love with someone else. Sister then later gives in to their brothers affection. (exception here-blood relation)

Both deal with a forbidden love between step brother and sister.

Both deal with a forbidden love between step brother and sister.

forbidden love but in boku wa.... it is between twins. both are great story one has a happier ending though.

A forbidden love between a stepsister and stepbrother who love each other.

A stepbrother and stepsister fall in love even though they are not blood related, their love is still forbidden.

They both have a similar tone when it comes to the relationships in these movies.
They are both about love that not a lot of people in society accepts: A Muse is about a relationship between a teenager (still in high school) and a much older man. Then Boku wa Imouto ni Koi wo Suru is about a brother and sister in love with each other.