This is never easy, when one person forgives and another in the family gets insulted by the act of forgiveness. I think the key issues here are these:
1) Be sure to acknowledge that your father’s leaving is morally wrong. I am presuming that your mother did nothing so egregious as to deserve this. Your brother might think that by your forgiveness, you are condoning your father’s leaving, which you are not because forgiveness does not condone wrongdoing.
2) Gently point out that forgiving is a free will choice by the one who offers the forgiveness. You are free to offer it and your brother is free not to offer it. Your individual choices do not make either one of you bad people.
3) Try to find common ground, such as your shared desire for your mother and father to be reunited. This common goal may help you to work as a team.
4) Finally, your brother’s refusal to forgive today is not necessarily his final word on the matter. Be open to change in him. If he becomes open to forgiveness, he might want and need to ask your forgiveness for how he responded to you when you forgave.