Section 60B of the Family Law Act states that children have a right to a relationship with anyone concerned with their care, welfare or development, which includes siblings.
Your niece should contact Legal Aid or Relationships Australia to organise a family dispute resolution conference with the father. This is a mandatory step before she can file an initiating application for parenting orders with the Court, and the intention is to try and get parties to negotiate an agreement together rather than going to Court.
If he doesn't participate, she can file for parenting orders from the Court. Of course, she isn't a parent, but 'parenting orders' is the blanket term that refers to any orders made in relation to children's care. As a person of significance to the child, that being her sister, she is allowed to file for parenting orders with the Court to seek regular time and contact with her sibling.