Legal Question in Wills and Trusts in Pennsylvania

So what? There is no law that says when someone has a will and leaves property to beneficiary A and B that beneficiary B gets a bigger share because A never saw the deceased and B lavished him with attention. There is a whole category of stuff in life called "that ain't fair" and this is one of them. If the others never saw uncle, then he was free to make a new will leaving all to attentive beneficiaries and nothing to the others. He chose not to do so and neither you nor I can question the wisdom or lack thereof in his decision assuming he was mentally competent and there was no fraud or undue influence caused by the inattentive beneficiaries.

Did the sister and brother die before or after uncle? What exactly does the will say about what will happen if a named beneficiary dies before uncle? How long did mother and brother die before uncle and is there a survivorship clause of any kind in the will? If so, what does it say?

If the will leaves all to siblings of uncle and siblings die AFTER uncle, then siblings' shares pass through their estate as per their wills, if any, or under the intestacy laws of the state where they lived at the time of their deaths.

If both siblings died before uncle then you have to see what the will says if anything. If the will is silent, then a partial intestacy would occur and (assuming uncle had no lineal descendants like grandchildren; if he did have children, grandchildren, etc. they would inherit). The children of the deceased sibling split - 50% to mother's children and 50% to brother's children - the estate is not divided per capita (Latin for by the number of "heads" or beneficiaries) unless that is spelled out in the will. Most wills provide for per stirpes (Latin for by the branch or root). If the will says the deceased person's share goes to their children, then the result would be the same - a per stirpes distribution with 50% to mother's children and 50% to brother's children.

But I have to back up here - you say uncle has grandchildren? That changes the picture. Was there any mention of children or grandchildren? Did he specifically disinherit them? How do the uncle's grandchildren factor into any distribution to uncle's siblings?

I think since you are not the executor of the estate then this is none of your business and that the executor of the estate should sit down with their estate attorney, who will review the will and distribute the estate to the proper beneficiaries as per the terms of the will. If there is any dispute about the interpretation (if the will is not clear) then the estate attorney should file what is called a declaratory judgment action and ask the court to interpret what the will says. if the will is clear, then the executor has to do whatever it says. . If somebody does not like the proposed distribution, he or she can object to the accounting which will need to be filed before the estate is closed.

The only basis to do what you propose is where people object and an executor decides to do a family settlement agreement because the executor does not want to waste money fighting in court and there is a reasonable case that can be made by the objecting heir. I don't see that happening here because your siblings are the executors and must distribute the uncle's estate as per the will; and (b) the grandchildren are not going to file any objections if they are getting 50% of the estate.