When I looked at immigration dept website for the list of relatives who can sponsor, then I found out the following:
The relative or the relative’s partner you want to sponsor must be your:
child or stepchild
parent or step-parent
brother, sister, adoptive brother, adoptive sister, stepbrother, step sister, niece, nephew, adoptive niece, adoptive nephew, step niece, step nephew, aunt, uncle, adoptive aunt, adoptive uncle
grandparent or first cousin.
Your sponsor should provide certain documents as listed in the document checklist
Where in this it states that you can sponsor your sister in law or brother in law? Can anyone clarify?