My thoughts on the situation:
#1) If you and all the other beneficiaries have received your funds, then I do not see it as an issue
#2) If something happens to your sibling, it would be up to his estate to figure it out, not you. Unless of course, you have agreed to be executor./Personal Representative of his/her estate (but even then you can, legally, refuse to accept the court appointment).
#3) I suspect, but am not certain, that if the funds sit there unclaimed for a while they would eventually be escheated to the state as unclaimed property. This does not mean that the state gets ownership of the property, but rather that it becomes a public record that this person has funds coming to him/her and that he could come forward to claim it or a representative of his estate (if an estate exists).
4) If the fund in question is a traditional IRA, then in general RMD's will be due from your sibling. If you Dad died in 2019, then the old laws on inherited IRAs would still be in effect (ie stretch IRA). Note that no RMDs of any kind are required in 2020 due to Covid-- you sibling would have until 2021 to take the first RMD. As such, there would be no tax penalties, IMHO, if this is not claimed before the end of 2020 Perhaps your sibling understands the law better than you think he does. My DMIL passed in 2019 and I am dealing with a similar issue except that we are talking about a 401k and the new administrator wants to release the funds to the estate, not to the beneficiary, disregarding the plan document rules that were in effect on the date of death, but that is another story.
Note I am not any type of professional with regards to this, but I have administered a couple of estates and like to understand the details.