The son.
The step son has no relationship to the aunt. So technically the niece would be the next of kin.
Mother, if you mean wife of the deceased husband.
If a person who has no spouse or children dies the living parent(s) would be the next of kin for purposes of inheritance. You can check your state laws of intestacy at the related question link below.
If you are speaking of the relative or relatives entitled to share in your property if you die intestate (without a will) then if you are unmarried your parents are your next of kin. If they were deceased then your sister would be your next of kin. In another sense they are all your next of kin. See related question link.
The son would be the inheritor of the remains of the estate in all the rules if succession that I have ever seen.
Without a will of intent all inheritance will go to the next of kin. The order of that is wife, father/mother, son and daughter.
Generally, for the purpose of making funeral arrangements and petitioning for probate: In the United States and most Western countries a surviving spouse has the legal right to make decisions as the next of kin. In the Unites States and most Western countries when a parent dies leaving no surviving spouse, all his children are his next of kin NOT only the eldest son. Primogeniture, or the right of the eldest son to inherit, is obsolete. It has been relegated to history where it belongs. In patriarchal cultures in the Middle East most favor a son or male relative. For inheritance laws you can check the laws of intestacy for your state at the related question link provided below.
next of kin would be eldest child
Generally, the grandchildren would be the next of kin. You can check state laws of intestacy at the related question below.
The state of Texas recognizes the surviving spouse as next of kin. For example if you want to obtain medical records and you are not the surviving spouse, you can obtain legal documentation (letter of Administration or Exectrix of the Estate), and this combined with a copy of the death certificate will allow you to acquire copies of medical records.
Do something nice for next mothers day
In the United States and most Western countries a surviving spouse has the legal right to make decisions as the next of kin. In the Unites States and most Western countries when a parent dies leaving no surviving spouse, all his children are his next of kin. As for inheritance, you must check the laws of intestacy for your jurisdiction to determine who inherits. It varies in different jurisdictions. Most pass the decedent's property to the surviving spouse. Some jurisdictions also provide a share to surviving children. You can check the laws in your state at the related question link below.