Probating a life insurance policy if the beneficary is deacesed and I am the contingent.

Who died first?  If your mother died before your father then in that case, as the contingent beneficiary, you would be entitled to all of the life insurance proceeds.  They would pass to you directly; that is they would not be considered to be a part of the probate estate.

On the other hand, if your father died first then your mother (now her estate since she is deceased) would be entitled to the proceeds.  Since your mother died without a Will she is said to have died "intestate".  Therefore the intestacy laws of the state where she legally resided at the date of her death would control just how her estate would be distributed.  Typically, it is to the surviving spouse, if any (not applicable here) and to children.  

Since I don't know whether or not your mother adopted your step-sisters it's hard to say more.  Additionally there may be other facts not set out here.  What you need to do at this point is to consult with an attorney in the area of where your mother resided when she died.  They can more fully apprise you of your rights under applicable state law.