@James Mc Ree, "Any thoughts on the status of this contract?" By the definitions you used for Void and Voidable, whether the contract is void depends on the Deed held by the parent and child. That information is not posted. However, if the Deed contained a right of Survivorship, then the child owns the property even without probate. Without the Right of Survivorship in the Deed, it would seem that your contract is void as to the interest of the deceased parent (who never signed the contract) and voidable as to the child (who we presume can not perform). But we haven't seen the contract so either. It is possible the child only contracted to sell his/her interest. None of that really matters. The sale will proceed only if you want it too. Under the circumstances that you described, there is almost no chance the seller will attempt to force your purchase. That puts you in the driver's seat. It is time to re-negotiate for better terms (price and/or payments).
I agree completely with the sage advice of Mr. @Daniel Smyth,
"You may be in a better place than you think you are. However, If I was the estate manager, a grieving family member, and felt that you were disrespectful with your eagerness to capitalize on the death of my loved one, I may be wanting to sell to anyone OTHER than you!
Caution. Patience. Send a card, be polite. Emotions are strong enough to destroy nations!"
It is possible for you to take their mess and make it your profit. You could go ahead and offer to rewrite your original purchase agreement, acknowledging the seller's need to complete probate and agreeing to $50/day until the seller can fulfill the sale. You could also offer to take the property off their hands, as is, via Quit Claim Deed(s) from everyone with a legal interest in the property -for a substantial discount on the price (25%)?
Then you can immediately rent out the property and take your time completing the probate yourself, or your could just rent the property for the next ten years, then clear the title via a Quiet Title Suit. (New PA law requires Ten years if the property is less than a half acre -see the affect of PA house Bill #352 lowering Quiet Title times via Adverse Possession to ten years for some PA residences.) At that time you'd be claiming full title via, adverse possession against any unknown heirs, who may have retained legal interest in the property as a result of it not having gone through probate.
https://www.legis.state.pa.us/cfdocs/legis/LI/consCheck.cfm?txtType=HTM&ttl=42&div=0&chpt=55&sctn=27&subsctn=1
James, if you decided to accept a simple Quit Claim Deed now, the key info you should have is: a copy of the current Deed, a copy of the will, and knowledge of the PA law on intestate succession. The current Deed specifies what interest the deceased parent had, and can control what happens to that parent's interest when he/she dies. Otherwise, the will controls what happens to the deceased parents interest in the property. If the deed does not provide for a right of survivorship, and there is no will, or if the will does not include the property, then the PA statute for intestate succession will control who inherits legal interest in the real property of the deceased.
Your are in the driver's seat. Just decide what works best for you and for the surviving child. There might be considerable profit in just taking the entire problem off the child's hands.