The Real Reason Cleopatra Executed Her Sister
Let's be honest: there are a million perfectly reasonable reasons for putting your little sister to death. Maybe she's borrowing your stuff without asking, or she ratted you out to your parents when you got home after curfew, or she unplugged your SNES when you'd just hit World 4 in Super Mario Bros 2. Maybe she instigated a violent revolution against you, killing a bunch of your boyfriend's employees and declaring herself Queen of Egypt. Maybe she stole your lip gloss.
In the case of Arsinoe IV, cherubic younger sister of Cleopatra, it was the violent revolution thing. And maybe also the lip gloss.
No Pharaoh-way? De-niled.
So what happened was, around 68 BC, Cleopatra's dad and an unnamed lady type had a wee baby girl named Arsinoe IV. This meant that there were three royal young 'uns in line for the throne: Cleopatra, Arsinoe, and their brother Ptolemy XIII, who was also 100% married to Cleopatra, because sometimes history is super gross. Once their pops kicked it, Ptolemy ousted Cleopatra, and Cleopatra buddied up with Julius Caesar and Game of Thronesed her way back into power.
Meanwhile, Arsinoe was keeping busy declaring herself the sole queen of Egypt, first hooking up with rebel general Achilas and then having him executed, placing her trusted right hand man in charge of his army. A quick attempted coup led to Arsinoe's capture by Roman forces, and she was sent to Rome to be part of one of Caesar's "look what I can conquer" parades. Her unique status as the sister of Caesar's main squeeze meant that she was spared execution, instead being sent to a temple to live out the rest of her days. Not that there would be very many of them.
A few years later, Julius Caesar came down with a nasty case of getting stabbed a bunch, and Cleopatra needed a new best friend. She found one in Mark Antony, one of Caesar's favorite dudes and the new leader of Rome. As recounted by the Dangerous Women Project, Arsinoe had seen a groundswell of retrospective support around the Mediterranean, and Cleopatra wasn't really feeling another possible revolution, so she asked her new boyfriend, who you may remember ran for office on a staunch stop-stabbing-royalty platform, to have her sister stabbed. And he did.