Why are 

 Sirius -  @callmesnuffles- 

Lupin -  @MessersMoony   

James -  @THEONLYJAMESPOTTER  

and

 Lily -  @LilyEvans_Official  

 all alive?

When Harry went to sacrifice himself to Voldemort to get rid of the Horcrux inside of him, he used the resurrection stone to bring these for people back. At that time, it did not bring them back in flesh and blood, but rather, a more delicate shade spirit. As we all know, he came back to life, but that is a separate matter. Afterwards Harry threw away the stone and got rid of it. Before this, Harry was the master of death. With all three of the Hallows belonging to him, that was indeed him. He, however, knew the dangers of some of the items and threw it away. His greatest inner desire was to have his family and loved ones back. Harry had in fact, however,  by throwing this away, became greater than the "master of death" as he was great enough to acquire them but greater to let them go never to be used by him or anyone ever again. With that, the resurrection stone actually brought back those 4 people.

With that, it is explained why Lilly, James, Remus and Sirius have been brought back to the land of the living.