First, let me say that I am have been a paralegal/legal assistant for a lawyer in Washington State for over 20 years, and we primarily practice in the area of elder law (estates, trusts, probates, guardianships, etc.). I am not sure where you are located, but I can answer this: Powers of Attorney become null and void when someone dies. That is a fact. Did the father have a Will? If so, then depending on how the father had his assets titled and what was stated in his Will, then those assets would pass to the beneficiaries, or a probate would need to be started. If he did not have a Will, then someone would need to petition the court to start a probate to administer his estate. If the ex (mother) and aunt take any money under the assumption that they have the power of attorney, then they are breaking the law and could be held accountable for the money they take (the children's inheritance). I can't stress enough how important it is to have a Will made to avoid these types of issues!!
Second, are the children (youngest siblings) adults? If so, then one or both of them could petition the court to probate the estate and then, depending on whether there was a Will or not, our state (Washington) has a "law of succession" which states that in the event that an individual dies intestate (without a Will), then the first person to receive the estate would be the surviving spouse (if any), then to the children, then to siblings, then to parents, etc. If the children are minors, then someone else would have to petition to probate the estate. Anyone can petition to administer an estate, however it is up to the court to determine who would be the appropriate person to administer the estate if the individual died without a Will.
I hope that helps answer your question. If you have any other questions, please feel free to message me!! Good luck!!