Succession conflicts between siblings are an endemic feature of monarchy. Primogeniture (inheritance of firstborn son) isn't a universal principle, but just a way of doing things popular in late medieval England and France as well as other times and places.

Just to throw out one random (there are very many) example of a younger sibling overthrowing an elder is Henry I's defeat of his elder brother Robert Curthose. They were both sons of William the Conqueror.

Even if they weren't effective, if you looked up any monarch with siblings, you'll probably find at least one thorn in their side. Gaston, Duke of Orleans conspired against his mother, Marie de Medici during the regency of his brother, Louis XIII. Gaston was the heir apparent for 20+ years. I think he plotted several rebellions and had to be reconciled with both Louis XIII and Richelieu, but ended up dying in exile. King John rebelled against Richard I while his brother was off fighting the Third Crusade. That rebellion failed and John just had to wait until Richard died to assume the throne. While fantastical in many regards, I find Game of Thrones pretty believable when it comes to the grab for power, court intrigue, plotting and family against family, brother against brother.