Where do I start?
Uldren is the Queen's brother yes, and he was by her side in Destiny 1, back when the Guardian was seeking information on entering the Black Garden. He was also by her side (near enough), when the Awoken fleet assaulted Oryx's Dreadnaught in Destiny 1 The Taken King. This is where Mara disappeared, presumed dead when Oryx unleashed his paracausal weapon, destroying everything in a fifteen-thousand kilometre radius around the Dreadnaught.
Uldren survived, crashing on Mars but with no knowledge of what had happened to his sister. He could still sense her however, and put all of his efforts into finding her again.
Meanwhile, The Reef was leaderless, searching for either of their ruling Sovs. In both of their absences, Petra Venj became acting Regent, and like Uldren, also spent her time looking for Mara Sov, not believing that she had perished.
Uldren was always ruthless, doing what he had to, to reach his goals. His mind strained, he let himself be captured by the Fallen House of Kings (supposedly approved by his sister), in a bid to help them in return for use of their resources and intelligence to find Mara. This made him a traitor to his people, the Awoken, and over time his associated with the Kings led him to start conducting raids on the Reef, killing and looting, justifying his actions by Mara's sacrifice. He also wanted to destabalise Petra's control over the Reef, as he saw her co-operation with the Last City as an insult, having held a dislike to Guardians for a long time.
When the Traveller awoke, Uldren broke. He could no longer hear his sister's voice, and started drinking Ether to numb the pain of losing her. At some point, he sensed her and followed it, coming across a near-death Fallen Archon Priest by the name of Fikrul who had been attacked by Guardians. Wishing to himself that he could save him, Uldren found his wish come true and Fikrul be restored to full life, but with one change; Fikrul's Ether was corrupted. Converting more and more Fallen into Scorn, Uldren and Fikrul took over the House of Kings, eventually converting it entirely.
At this point, Uldren leaves the Scorn to their own devices, concentrating once more on Mara. He tries to talk to Petra Venj, but realises that wouldn't go anywhere. After a vision of his sister, Uldren tracks down a Techeun, Ilyn, and accuses her of keeping Mara Sov trapped in the Dreaming City. While on a rant about what direction the Awoken people were heading after Iltn accused him of madness, Uldren has a moment of clarity and gives up, surrendering to the Awoken.
Seeing that there was clearly something wrong with him, an infection of Darkness (presumably from the Battle of Saturn) but not willing to look past his crimes against the Awoken, Petra and Cayde-6 throw him into the Prison of Elders, where he wouldn't cause any harm.
Then came a prison break.
Uldren, fully convinced he was following his sister's wishes, left the Prison with the Scorn, killing Cayde-6 in the escape. Having been told by his sister that she was trapped in the Dreaming City, he made a key out of Light (a shard of the Traveller), and Dark (from the infection in his body). Racing against the Guardian, he got to the Dreaming City Watchtower uncontested to meet and free his sister.
But there was one small problem. Ever since the Battle of Saturn, he was never having visions of his sister or hearing her. Instead, it was an Ahamkara, a Wish Dragon, that had been communing with Uldren all this time, and while he detected that something was amiss, his blind love for his sister made him keep on going. The Ahamkara, Riven, had been brought to Mara Sov by Uldren when it was just a baby, and Mara created the Dreaming City using Riven's wishes.
This was all undone when Oryx attacked the Awoken fleet at Saturn. When Oryx used his weapon, at the instant that it touched Mara, Oryx discovered the Dreaming City and visited it. The corruption of his prescence introduced the Taken, and Oryx also Took the Techuens and Riven. Riven was tired of being trapped in the Dreaming City, even after Oryx's death, and so, started to parasitically influence Uldren's mind to achieve escape.
The worst thing about all of this for Uldren?
His sister had always been alive. In the same instant that Oryx vapourised Mara's body and discovered the Dreaming City, Mara became a soul lost on the Ascendant Plane, much like Toland the Shattered. On Oryx's death, Mara was strong enough to find what she needed in the Ascendant Realm to free herself, allowing Petra to commune with Mara with the Oracle Engine and prove that she lived still to Petra and the Guardian. While she was sad about her brother's death after the Guardian's battle with the Voice of Riven, she did not hold a grudge.
Bit of a lengthy reply, but that's essentially the basics of Uldren/FakeSister/Sister situation between Destiny 1 and now. With the Forsaken lore collectables, there's a massive amount of lore for the time period before Destiny 1 involving the Sov siblings, such as when the Collapse happened (creation of the Awoken), Uldren's first visit to the Black Garden, and so on.