1) What can I inherit from my PARENTS and what can I inherit from my HUSBAND?
You are entitled to inherit your legitimate share in the property left behind by your parents who died intestate, but nothing during their lifetime.
The same is applicable in your husband's side too.
Even if you convert into Hindu, your rights in the intestate property of your father is protected, you can avail it as a daughter to your parents.
2) What will my husband inherit from HIS FAMILY, and what can he inherit from ME?
Whatever he is entitled to from his family side i.e., a share out of his father's share in the ancestral property, a share in the properties let behind by his parents upon their intestate death, etc.
He is not entitled to any share from your properties during your lifetime.
3) If my parents leave a will giving their property to my brother, will I still have any share in it or will all of the property go to my brother only? (This question is most important)
As per Muslim personal law or sharia law, A person cannot bequeath his entire property through will to any beneficiary. Only 1/3rd share of property ca be bequeathed through a will, hence you will be entitled to a share out of the balance 2/3rd share of property.