Sister Sister ~Koi no Heartful Panic~

Main character lives with his brother and girl Futaba brought by his father. They grow up as a happy family together. But everything changes when Futaba receives an unsigned love letter containing only "I love you" line. At the same time new transfer student also appears to be their distant relative and thus also an older sister. Brothers decide each to choose his sister, but the decision won't come without a conflict.

[From vndbreview]