To sin against the holy spirit is not to sin against a person other than the Father, but to sin directly against the Father, because the holy spirit is an extension of Him that exercises power over whatever God wants to act upon.
We know that the holy spirit is the same as the Father (His extension) because when Jesus mentions forgivable sins, he mentions the Son of God, but does not mention the Father... It is because to sin against the holy spirit is to sin directly against the Father.
Lk. 12:10 And everyone who says a word against the Son of man, it will be forgiven him, but whoever blasphemes against the holy spirit will not be forgiven.