Hello and thank you for your question.
Even had your husband survived he is not legally entitled to expect to inherit something. Under Australian law people can generally leave their property to whoever they choose, and whilst most people leave property to their children, that is not always the case.
That said most states have laws that allow persons who are actually dependents to others to claim against an estate if they are left out, but they will generally only succeed if they can satisfy the court that the deceased ought to have made some provision for them having regard to other beneficiaries. If your in-laws provide you with regular support prior to their death you may be able to satisfy a court that you are a dependent who ought to have been provided for, as a basis for later challenging their wills if no provision is made to you, and this may be the best way to establish a legal claim, but their support would have to be voluntary.
In short, you can ask your in-laws to make some provision for you, but there is no legal basis for forcing such an outcome.
I appreciate this is not the answer you were hoping for but it does reflect the reality of your situation.
Good luck and please rate my answer.