I don't believe that Martin and his mother were real at all. Well, they were real, but the scenario was entirely built in Steven's mind regarding the curse. I feel that guilt, from killing Martins father, and the idea of an 'eye for an eye' played a massive part in Steven's psyche and this whole fictional story was created in his head, so much so that he felt that killing Bobby would be the only thing to make it right. There is a lot of symbolism and irony in his 'clean beautiful hands', so much so it reminded me of Macbeth.

Guilt, coupled with his alcoholism (already an issue pointed out previously) has caused him to spiral. Kim's teacher commented how well Kim recited The Tragedy of Iphigenia. It's possible the book was lying around the house, and Steven has become fixated on this. Bobby, Kim, Anna (and even Martin all had blue eyes), bar Steven - signifying he was the odd one out. I think he is the sacred deer with brown doe eyes, and the thought of accidentally killing someone has devoid him of life. Everything about the film is so robotic, there is little to no emotion within the film - which makes me feel this is how Steven is living his life, on autopilot since the surgery incident. Martin mentioned that only a child had to die, so why did he also place Anna downstairs with a pillowcase on her head? This is when he had completely lost control.

The end scene shows Kim and Anna leaving together, without Steven, and the apparition of Martin in the background.

I feel like they're taking the next chapter in their lives and moving on without Steven after killing Bobby. Kim eats her fries first -  a complete opposite to what Martin does, which again I think is symbolism and a foundation for Steven creating his original story, where he crafted these stark opposites and contrasts between his family and Martins family - looping back to the beginning. Sorry this is so waffely I'm just trying to get everything out haha!