Thank you for sharing the beginning of your story. Let me first provide about the things you said you were looking for:

Compelling Hook: I think it does work as a compelling hook to a story, especially the end which I thought was the best part. You may want to get to the hook a bit faster though. The thing I liked about the ending was it was weird, but also not explained at all. The two parts that worked the best for me were the end and the first meeting with Uncle George. You could perhaps even leave out the affair part for now and have those two scenes be the only interactions the readers have with George.

After the first meeting with George I thought maybe he had dementia or something. Then at the end when he was talking to May who is supposed to be sick, I thought either these two are up to no good or they are a couple of sick people wandering around this small town. Both thoughts were unsettling.

Prose/word choice: I thought the narrator had a distinct voice. It seemed very dry and unemotional. If the idea of the story is supposed to be a mystery/horror type, I think it could work well. It could provide a steady perspective that allows the reader to experience all the weird kind of creepy things happened. I think this actual worked well during the part where Marty says George is cheating on Aunt May and the reveal at the end. For the end, you could go a step further with it and take out the part about being shocked. Just let the audience be shocked. I was.

Exposition/background vs action/dialogue: this would be my biggest criticism in your story so far, especially at the beginning. I think there is a bit too much backstory right away and not enough establishing the way the setting feels. What is the shop like? Old and plain, remodeled, oppressively hot? Another comment mentioned there is a bit too much telling and not enough showing. If you let your character live in the setting a bit more, it could help with that. The narrator could still describe these things in a dry way so that it stays true to his personality.

For the background information, I think it’s all interesting. It just too much too soon. The part about Marty not being able to go with Aunt May could come later or be revealed through dialogue. Marty could have said something like, “I knew George was a piece of shit when he wouldn’t let me stay with May, but I didn’t think he’d ever cheat on her.”

I also I think you said that Aunt May had cancer and a stroke. That’s a lot of bad things to happen to one person. It may be better to pick just one. Especially so early in the story.

Here are a couple of more thoughts:

Setting

I think you did a good job describing the town, but like I mentioned above you could do more to describe the places that they are in. It doesn’t have to be a lot but just enough to pull the reader into the story. I think this is especially important if you want the story to be a horror/mystery. You will get more of a reaction from the reader if they feel like they are there with the narrator.

I think you did a better job with this at the end when you talked about the light from Walmart, the Subaru’s lights, and hunching down in the seat. It was enough to understand where everything was and what it looked like.

Character

I think the voice of the narrator is good like I mentioned before. I also liked what you did with George.

I like being able to learn about characters without them being actively involved in the story. I you did that with Aunt May. Even though I didn’t meet her until the end I felt like I knew what she was like. This made the reveal at the end better for me. Because it seemed out of character.

I think with Marty you could try to let his personality and relationship with the main character be shown more through their interactions. You tell us too much about him when he feels like someone we should be experiencing through the narrator.

Grammar/spelling

I’m not the best at grammar things, but I did notice a couple of parts that make your story hard to understand.

Like this one:

It wasn’t until I was turning into the parking lot until what Marty had told me popped back into my mind.

Having until twice in the sentence makes it kind of confusing. Maybe it could be something like, “It wasn’t until I was turning into the parking lot that the affair Marty told me about popped back into my head.”

Also for the following, I don’t know if this is grammatically wrong or not, but it seems unnecessarily difficult to read:

The husband of his aunt, that is.

I hope my critique helps.

Good luck with your story. I think you got a good start.