I hope you will understand my English: I read this language but write it very little. I am writing this text in French (I am a Quebecer) and I am having it translated into English by Google Translate, excuse any errors, please. You sow a lot of clues about Tyler's origins and similarities to Kyle, and I believe that, if we analyze these carefully, we can conclude that Tyler is, at least, a half-brother to Kyle.
I have some speculations but I need more information.
Did Colleen, Tyler's mom, cheat on her husband and become pregnant with Tyler after dating another?
Or did Colleen have artificial insemination (maybe from David)?
Did Helen have an affair with Sam, her first love, and, having given birth to Tyler and deeming herself incapable of raising him on her own, gave him up for adoption to Colleen and her husband?
I hope to get some feedback from you, an author whose subtlety I admire, and from the readers of Tyler's story.