Dream and nightmare sans x sibling reader

Prologue

Long ago in Dreamtale there was a apple tree that was called the Tree of feelings. The golden apples holds positive feelings: happiness, hope, gratitude, entusiasm, faith, piety, etc.; while black ones holds negative emotions: hatred, resentment, fear, depression, wrath, etc. Dreamtale had the tree of feelings that produced negative and positive feelings, that were sent throughout each alternative universe (you can call it an Au if you want) so life could naturally keep subsisting. 

Without the tree, no being throughout the multiverse could ever feel anything, like a soulless husk without any kind of emotion inside of them. However through time people who obtained negative feelings than what they should have gotten, have intended to do damage to the tree in order to obtain the golden apples, since not only that they have power, if you have any of them on your side, they will grant you enough confidence to do any kind of job without taking any failing risk, even if the job was that bad, like killing someone.

Before the mother guardian gave birth to Dream, Nightmare, and (Y/N) one of the bad people was able to severely hurt her, whose life was connected to the tree itself, that means that if she died, the Tree of feelings will die with her. 

After the guardian killed the aggressor, she started to feel repulsion towards the mortal being. She had one choice so that they wouldn't die was to fuse with the tree, and she would save the life of it, but if she did that, the tree wouldn't have any guardian, and the fruits would be stolen within easiness.

The guardian became scared from all the blood loss, and she decided to use to magic of the tree to bring all the positive and negative energies that she could to creating with these life.

Three light beings made with pure positive, negative and hybrid energy, the little golden being was known as Dream since he was a dreamer, the dark purple being was Nightmare because he was the complete opposite of Dream, and their sister/brother (Y/N) since he/she was half monster and human and combine both positive and negative emotions. The guardian gave the three bodies so that they could take them, empty bodies. But the energies were so big, and since there were a few escape routes, the bodies began to swell up more and more, until they exploded. The guardian had to think fast, and she had to fuse soon as possible. So she explored the multiverse with her mind to find some vessel that she could copy and that it was suitable for the guardians. She has found a small skeleton in a Au called "Underswap", his name was Sans and a hybrid human/monster from a Au called "Axetale", her name was Aliza.

So she hurried and copied the form of that skeleton and hybrid into the three empty replicas in front of the light energies, who have taken the bodies quickly, that filled them with life. But this time, the energy have made escape routes due to the numerous holes that a skeleton and hybrid could have, so it was under control. The guardian (that is now the mother of the new guardians) fused with the tree. 

The two small skeletons and hybrid didn't have anybody, they passed the days talking to each other. they were siblings, they were always together. Even to cover the blood stain that the tree still had dried. They decided to create a yellow flag with the sun (that was Nightmare's idea) and tied it to the tree. Everything was in happiness for all three of them, after all, nobody approached to the tree.

But as Dreamtale started to get filled with more and more life they created towns around the tree... and everything changed for the worse.

The skeletons and hybrid finally were alone, and sometimes they hear the voice of their mother echoing as if she was still with them. They had each other, and slowly everything started to get filled with more and more people. A huge town was being build near the tree. Due to the newest condition of the world to other beings, that everything was being to rustic, without any technology and no kind of entertainment beside of the land work. the kids were cooperative, but not everything was colorful. In every society there's always people... who are willing to hurt others.