Luke's eyes widened and his jaw dropped, his whole body jerked a couple times as he processed what he'd just heard, and tried to get the words out, "My...my, brother?"
Han flinched as he anticipated what was going to come next. Vader had summoned them both to his office and he had told Luke the truth, his past as a newly married teenage father, the first baby they had given up for adoption, his suspicions, and finally, the confirmation that the man standing alongside him was his older brother.
"Yes, Luke," Vader answered.
Luke's whole body made various wracking movements as he tried to make sense of what he'd just heard, and Han was already dreading the fallout. He expected Luke to start screaming, or crying, or storm out of the room, whatever was about to happen he knew it wasn't going to be pretty. He wasn't even sure what to say.
Luke turned to him, and he stood there, his whole body swiveling for a moment, looking like his brain had just short-circuited, and Han braced himself for what happened next. Luke let out an incoherent yell and he charged at Han, who expected the kid was going to punch him in the face, so he was more than caught off-guard when Luke wrapped his arms around him and hugged him, he'd be damned, the kid actually lifted him off his feet with his momentum. It dawned on him the kid was actually laughing. Luke grabbed two fistfuls of the front of Han's jacket and clung to him as he threw his head back laughing victoriously, but a few seconds later he slumped against the captain as his strength left him and he fell to his knees crying. Han kept a hold on him and knelt down alongside him to keep him balanced. Luke looked at him with tears welling up in his eyes and already trailing down his face as he asked him, "You're really my brother?"
"Unless that droid screwed up the blood test," Han offered, not sure what else to say. He wasn't sure how the whole Force thing worked but he was pretty sure Vader was able to use it to reach across the room and cuff him upside his head, his eyes bugged out in response to the sensation.
Luke dropped his head on Han's shoulder as his whole body wracked again, Han held his brother in his arms to try and steady him.
"I..." Luke's breaths came out in wheezing gasps, "I always, wanted a brother."
"Same here, kid," Han told him. "Same here."
Well, that actually went better than he'd thought. He just hoped this wasn't the shock and there was a more serious delayed reaction coming later.
Darth Vader found himself walking a fine line between respecting his son's wishes and trying to establish the family he wanted. Han was adamant he wouldn't quit the fleet, and he didn't want people to know Vader was his father. He'd fought and struggled and earned everything he'd ever gotten, he was determined to keep it that way, a quality Vader could strongly admire in his own child. They'd reached a compromise however, Vader had had one room on the second floor prepared as Han's own quarters, though he strongly suspected he knew where Han would be staying the nights he was on leave.
As a matter of fact...the dark lord headed down the corridor and opened the door to Luke's room and peered in.
The two of them were asleep in Luke's bed, this time each on their respective own sides. The family resemblance wasn't clear between the two of them, but both looked like they were precisely where they belonged.
His sons. Both of his sons. He never thought this day would ever come. He remembered shortly after Luke was born, imagining what it would've been like if his first child was still with them. He tried to envision what his oldest son would look like, tried to picture him peering down into Luke's crib as he slept, meeting his younger brother for the first time.
Over 25 years he'd relived that day and that decision a million times. Every time he knew there had been no better solution, and hating himself that that was true. The birth of their child had jolted him and Padme out of whatever naivete they'd looked at her pregnancy through. The reality hit they were both young, inexperienced as a married couple, far less experienced and terrified out of their minds as parents, and had no idea how they were going to raise this child for 18 years. They'd kept him for two days to make sure he was strong enough to survive, and to make sure this was truly what they wanted to do. Well, that was false, neither of them wanted to give him up, but they knew someone else could give him a better home than they could.
Padme was still weak from the birth and had to be kept in the med bay, Anakin waited until she was asleep, he wrapped the baby up and had taken him out, carried him in his arms as he walked to the temple and gave the baby to one of the priests. The whole way back to the med bay, he felt like he wanted to die. For a week they were both so heartsick that Anakin wasn't sure either of them would survive. In spite of their grief, life went on, one day became the next, became the next, and they were forced to resume their otherwise normal life.
It took so long for Padme to get pregnant again with Luke, for a while they both considered the dreaded possibility that perhaps the child they had given up was the only one they were meant to have. When she finally knew she was pregnant again, they were determined to do right by this child, but nothing could negate the guilt they felt at giving up their first son, having no idea what had become of him.
Luke was two when Anakin Skywalker became Darth Vader. He remembered waking up in the med bay in his new durasteel body, the events leading up to it were a blur to him but slowly returned. The chancellor Palpatine had killed Obi-Wan on Mustafar, Anakin had tried to avenge his friend and teacher's death, Palpatine had in turn used the dark side powers of the Force and thrown him into a lava pit. However, Anakin was determined to take the sith down with him, he grabbed hold of Palpatine's robes and his waxy corpse-like neck and they went into the lava together. He remembered Palpatine's agonized screams and the sickening smell of his burning flesh, before his own joined him. He resigned himself to the fact he would die, but at least Padme and Luke would be safe, and for a while anyway, so would the galaxy. When he came to in the med bay he realized most of his body had been burnt beyond use or recognition, he had to learn how to use durasteel limbs and how to move them as effortlessly as he had his own flesh ones as he also adjusted to life on a respirator, in a life support suit. It was a way of life he'd never had to consider before.
Padme was there. She kept her distance so the medics and technicians could work on him, but he saw her face in the window outside his room. On occasion he'd look over and saw her holding Luke up in the window so he could see his father. Thankfully Luke was young enough his father's transformation didn't really register with him, or the boy would've been traumatized beyond belief.
When he'd finally mastered his new body and learned how to live with it, Padme and Luke came into his room, and it was then he learned that Padme was responsible for him being brought there. She'd argued with the medics and the doctors to do whatever was possible not just to save his life, but to rebuild his body so he sustained an actual quality of life. He found himself questioning just what that quality of life consisted of, as he was now he couldn't see his wife or child with his own eyes, only through red tinted lenses, he couldn't kiss his wife or his son, he couldn't feel their skin against him, it took a long time to adjust to these newfound facts and until he did, he felt like he was standing outside himself watching his life from a distance. While this was going on, he and Padme had a lengthy discussion about where things stood. They decided it would be safer if everybody thought Anakin Skywalker was dead, in this life support suit, hidden under a helmet and a mask, nobody knew who he was. He could claim a new identity and emerge as a new member of the Empire, once everyone learned that he had killed the mighty Palpatine, there would be few people who would press their luck trying to kill him. From that position he could work to overhaul the entire system and bring an end to so much pointless conflict that had plagued the galaxy for so many years. In theory it made sense, but Padme would never live to see it achieved.
Anakin Skywalker was dead and Darth Vader was born. He appeared out of nowhere and established himself high on the Empire's food chain, overnight he had asserted himself as an enforcer for the higher-ups; he also publicly claimed for himself the widowed Padme and her son, Luke. They lived this way for three years before Padme got sick. The scans showed a blood disorder that was very rare and without cure or treatment, her condition deteriorated quickly, and in a few months, Vader found himself with a son to raise, and a stone marking Padme's grave. Once again he found himself thinking heavily of the child he had given up all those years ago, where was he? What system was he in? Was he still alive? Was he alright? Their firstborn was the only other part of Padme that was still alive and out there somewhere.
He knew he could tear the whole galaxy apart and he'd never be able to find his oldest son. Instead, in the wake of Padme's death, he devoted all his time and resources to making sure Luke had everything he needed, admittedly he was spoiled for it, but Vader wanted to make sure Luke never wanted for anything, having already lost one parent, and not even knowing he'd had an older brother. He did everything he could to compensate for those blaring gaps in his younger son's life.
And now, now, finally, he had both of his sons together. At last, their family was as whole now as it ever could be. Looking at the two young men as they slept peacefully and unaware, the dark lord felt a swelling sensation in his chest. Finally, after 25 years, he'd finally found his firstborn son, he got to know him, what had become of him, got to see the fine man he had grown up to be. The pain of giving Han up was still there, and Vader suspected it would never truly leave, but he also felt a confirmation now, he saw how Han had turned out, and he knew, devastating as it was, he and Padme had in fact made the right decision.