At first glance it would appear so. Saradomin may be Guthix's brother, but as Guthix's sex is not known and Zamorak was a Mahjarret before a god, we cannot say whether or not Guthix is a brother to Saradomin and Zamorak is a brother to neither.
more explainable answer------
I believe no. They are not brothers. Saradomin Guthix or Zamorak was never linked to be brothers. They are just Gods.