Though Renly is Robert's youngest brother, Robert gave the Baratheon seat of Storm's End to him rather than Stannis, who was given Dragonstone instead. During the Targaryen dynasty, the seat of Dragonstone was given to the heir to the Iron Throne. Stannis conquered the island, but not before Viserys and Daenerys Targaryen escaped, which angered Robert. The lands of Storm's End are considerably larger and richer than those of Dragonstone, giving Renly more power than his older brother.
After Robert's death, through his charisma and familiarity with his Stormlands bannermen, Renly manages to win support from many houses, despite his brother Stannis having the better claim. Renly also won the support of the powerful Tyrells and many of their bannermen through a secret love affair with Loras and a marriage to his sister Margaery. Renly crowned himself King of the Seven Kingdoms, with his claim to the Iron Throne backed by the largest army in the War of the Five Kings.
He is the brother of Stannis Baratheon and the late King Robert Baratheon. He is married to Margaery Tyrell and Son-In-Law of Lord Mace Tyrell. He is Uncle to King Joffery Baratheon, Prince Tommen Baratheon, and Princess Myrcella Baratheon.
The Stormlands will declare war on the Westerlands and Dragonstone fairly early on, remaining on-the-fence about the Northern menace of an army that may eventually become a problem. Although King Renly Baratheon seems a potentially unstoppable force by the time House Tyrell joins his cause, thus supporting his self-proclamation of Kingship against the Throne even further, if the player is against him then it is actually not that hard to put a quick end to his claim and power.
War with the Stormlands will be mostly futile, as they prove to quite commonly rebind back to their feet after being knocked down. If the player wishes to completely end these futile strikes, his best chance is to keep Stannis Baratheon alive and free without war for a good time so that they may access assassination quest that allows you to execute Renly Baratheon should you capture him. This is fairly difficult, though, as capturing a King means you'll be commonly looking for him, and he will quite easily be able to hide from a lost battle. Renly also seems fearful of his imminent death and in a shock offers numerous states of power to his executioner should he be given mercy. As of now, the questline is broken but expected to be fixed by version 2.2.