Estates that go through probate can do so easily if the estate planning was done well; probate courts can do basic tasks like identifying assets of deceased individuals, determining the payments that need to be made for taxes and other expenses and assigning executors in uncontested situations relatively expediently. However, there are many cases where challenges arise, and probate litigation is the result.
Siblings may cite specific reasons for their estate disputes after their parents pass away. Maybe one sibling thinks that the other used undue influence to make the will say what they wanted it to say. Maybe they worry that the will is actually a fake, created by a sibling who was physically closer to the parents or that they tricked their parents into signing it.