Can you marry a step-sibling, in-law, or an uncle in Australia?

A mother of a bride asks one of our members:

I understand step siblings even if they did not meet until later in life cannot marry I got married a few years ago. My daughter and my husbands son met at an event we held, fell in love and now want to marry. I have recently become aware that under the marriage act they are considered to be related If I divorced my husband and was therefore no longer married, would I be considered a de facto? If so our children are not related by marriage or as defectors would they still be considered related? Given these circumstances could they marry? Would really appreciate your advice. My daughter is excited about her engagement and she will be crushed if she cannot marry. I’d be happy to divorce if it meant she can marry. My husband and I have had our wedding and it would be no sacrifice for us. I just need to know that it would not be in vain.

Who can get legally married in Australia?

To be legally married in Australia, a person must:

not be married to someone else
not be marrying a parent, grandparent, child, grandchild or sibling
be at least eighteen years old, unless a court has approved a marriage where one party is aged between sixteen and eighteen years old
understand what marriage means and freely consent to marrying
use specific words during the ceremony
give written notice of their intention to marry to their celebrant, within the required time frame.

Gender, religion, citizenship, place of birth, politics, skin colour, wealth, do not play a role at all.

Can you be related and get married in Australia?

The following relatives can not get married by lawe in Australia:

a person and an ancestor or descendant of the person
a brother and a sister (whether of the whole blood or the half-blood)
a person who is or was an adopted child, and, for that purpose, the relationship between an adopted child and the adoptive parent, or each of the adoptive parents, of the child, shall be deemed to be or to have been the natural relationship of child and parent.

Which basically translates to this: you cannot marry your parent, great/grandparent, child, or great/grandchild, brother or sister, blood or adopted.

So the other side of that list leads you here. Which relatives can get married in Australia?