This is my take on harry having a twin. Lily is alive though James does not survive the attack.

Lily loves the twins but she has a hard time taking care if the both of them so she asks Remus for help. Remus agrees to help his old friend and quickly begins to love with the two boys. The twins begin to see him as their father. The twin, Harry and Aden, were inseparable they did nothing without the other. Aden even avoiding speaking in front of their parents until Harry was able to speak as well. Though they tried not to let anyone know it Aden was almost always the first in everything. He was the first to be able to speak first to walk. There was one thing Aden was unable to hide until Harry caught up and that was he was the first to do accidental magic. At this point Albus Dumbledore took interest, and tried to insist that he be taken off for training. Lily and Remus automatically told him to… well it was not nice so I'm going to just tell you it meant he would never separate the twins. Needless to say Dumbledore was angry that he did not get his weapon. But he figured he would wait till they got into Hogwarts to try to get Aden.

As they grew the twin became even more inseparable. They did everything together. As they grew they also grew closer to Draco. The three quickly became best friends. On the twins seventh birthday they refused to do anything until Draco got there. The three were close but Harry was close to Draco than Aden was. Harry was almost as close to Draco as he was to Aden but there was a difference that neither could figure out but they figure it's not important. On Draco's seventh birthday, which was about four days after the twins' he decided that he wanted to spend the day with Harry and Aden but they insisted that he spend it with his fathers, Lucius and Sirius, so they compromised and Harry and Aden spent the day with Draco Lucius and Sirius.

It wasn't till the age of nine that the twins' interests stated to differ. Aden was more into the sports and flying and defense. While Harry loved to read and his favorite topic was defiantly a potions. In spite of their differences they were still as close as ever. Draco was did like flying but his true passion was potions as well.

The boys get there Hogwarts letters just after Draco's eleventh birthday. The boys were so excited that they immediately ran to show their respective parents.

Harry and Aden

"We're very proud of the both of you, but we want you both to be careful and don't trust a man named Albus Dumbledore. He wishes to separate the two of you and use Aden for his on gain." Remus stated with concern.

"We understand dad" the twins said in unison.

"Your dad and I proud of you and we love you both but we are just worried about you" Lily said to her two sons.

"We love" Aden said.

"Love you both too" harry finished.

Draco

"We are very happy for you son" Sirius said.

"Yes son but we want you to be careful of the headmaster of the school and keep an eye on the twins because they may be in danger" said Lucius.

"Yes dad I understand" Draco said.

"And remember Draco no matter what house you get into me and your dad still love you" Sirius said as pulled his son into a hug.

" I love you both to mummy" Draco said as he always saw Sirius as his mum and Lucius as his father.