OVERALL:
I think the characters are somewhat interesting, but could be more engaging/relatable. I don’t really feel much atmosphere of unease. I think the story has a lot of potential, although I feel like I don’t have an indication of which way this story is going, which is something I look for in first chapters. What promises are you making the readers? What are the emotional stakes for Sarah? Yes, her bro has jaw cancer, but I feel like you could draw out her feelings a bit more. What else is she stressed about? What other factors are at play in her life that aren’t on-screen here? You almost hint at that toward the end, but I’d like to see more of that sooner. What is Sarah worried will go horribly wrong?

THOUGHTS AS I READ:
* Personally, for asides, I prefer em-dashes (Alt + 0151 = — ) rather than parentheses. Parentheses don’t create enough of a idea break for my brain.
* When Brandon gives the pain scale, I’d flip-flop the phonetic and the quotation phrases, because Sarah’s interpreting the sounds for the reader.
* I’d refer to the smileys as “it”s rather than gendering them, since the wall chart doesn’t say it’s a guy smiling.
* You use a lot of words ending in -ly throughout the story. I think they bog you down some, and would cut at least a few +/- rework sentences to reduce the number.
* You also have a lot of exposition/infodump between the dialogue bits, enough that I forget there’s a conversation going on. I would consider trying to work some of the info into the dialogue, with the intent of the conversation driving the info reveals forward.
* There is a lot of past perfect tense (“had been”, “had seen”, etc.) and passive voice (any form of the verb “to be” – is, was, are, were, etc.). I think you have room to rework some of the sentence structures to make them more vivid (i.e., “had moved Brandon” to “moved Brandon [X weeks ago]”, etc.

Hope this helps, thanks for sharing!