Quick answer: 0% spouse/young child from a parent, 4.5% other lineal heirs, 12% siblings, 15% everyone else. Run your own numbers in the Pennsylvania inheritance tax calculator.
The four classes, straight from the statute
Why siblings pay more than children
Pennsylvania's 4.5% lineal rate applies strictly to the direct family line — children, grandchildren, parents, and grandparents. Siblings sit in their own separate class under §9116, taxed at 12%, regardless of whether they share two parents, one parent, or were adopted into the same family.
Stepchildren generally count as lineal, with a limit
Stepchildren and step-descendants are generally treated as lineal descendants and taxed at 4.5%, without needing formal adoption. But a step-relationship outside the direct parent-child line — a step-sibling with no shared biological or adoptive parent, for instance — can still fall to the 15% catch-all rate.
No recognition for unmarried partners
Pennsylvania does not recognize domestic partnerships outside of legal marriage for inheritance tax purposes — an unmarried partner pays the 15% rate even after decades of cohabitation, with no lower-rate category available regardless of the relationship's length or nature.
The 0% child rate is genuinely recent
For deaths on or after January 1, 2020, Act 13 of 2019 set the 0% rate for transfers from a natural, adoptive, or stepparent to a child age 21 or younger — a relatively recent addition to the exempt list, and one that runs both directions between that parent and that child.
A local probate attorney can review your estate — many offer a free consultation.
See Pennsylvania Inheritance Tax Exemptions for the asset-specific carve-outs — life insurance, family farms, and more — that apply on top of these relationship-based rates.