Representative of the estate is not the same thing as beneficiary of the 401k plan.

This means that his sister will have the job of making sure all of the assets get to where they are supposed to go. It does not necessarily mean she is entitled to anything.

For a 401k plan, the plan administrator, which is normally a person at the company, will have a beneficiary form on file. You could contact the company to see if they have one. If so, that form will designate who the beneficiary, or beneficiaries, are.

If there is not such designation because his father never filled one out, it will be up to the estate to sort it out based on state law.

If you have paternity results to prove parentage and you're sure your son is his only child, then you have to notify the administrator of the estate to that fact. Depending on your state's intestate laws (death without a will), even that 401K plan can be challenged if left to another beneficiary. The questions are numerous such as was he paying support? If so the child is eligible for survivor benefits from social security. Does the father's family know the child was his? You may have to produce enough evidence to convinced the court and the family to lessen their objections, which could cost you money. Since the child is a minor? If so, the money may have to go into a trust depending on your state's laws. What you need is a competent attorney in probate law who should take the case for a percentage of the estate he can get through the courts and state laws. Like I said too many questions to nail something down and in answer to your question, the sister doesn't get squat until the court says yes or no. Go get that attorney.

The sister wouldn't specifically get the 401(k) unless she was named the beneficiary of it. The named beneficiary is the person who receives the funds.

The estate might get the money, and any debts that the father left behind would have to be paid first. Then, depending whether there was a will or not, the remaining estate gets distributed to the legal heirs. Being named Personal Representative (which suggests there was no will) means that the sister is the person who will work with the probate court to identify the deceased's assets, liabilities and heirs.