If mother died without a Will was it legal for my stepfather to claim the entire estate?

I am so sorry for your loss.  When a person dies without a Will they are said to have died "intestate" and the intestacy laws in the state that they live will decide how the estate is distributed.  And although you may be correct as to the percentage of the distribution of her estate to you as a surviving child, it is important to remember that sometimes there is no estate when assets are held jointly as husband and wife.  If there was a house with "rights of survivorship" then the house passes automatically to the surviving spouse on death.  Same for joint bank accounts.  Life insurance policies have the ability to pass "outside the estate" with a validly executed designation of beneficiary form on file with the insurance company.  Those proceeds are then paid directly to the beneficiary listed.  If none is listed then they would go to the estate and then you would receive a position of the proceeds in the estate proceeding.  Yes, it is worth discussing the matter with an estate attorney.  And explore the possibility that there was a Will that your step-father chose not to probate.  See if your Mother's friends knew anything about her requests or intentions and see if a family attorney would know anything about it.  Good luck.