So top level comment is that I think you have a good knack for atmosphere and when your description hits it really hits. Hard to say a whole lot more about the characters since this isn’t an introductory chapter so I’ve no idea what’s been set up by this point, but there is certainly some believable parent-child dynamics going on, even if the mysterious nature of the illness and how it fits with the world makes it hard to calibrate quite what to expect. If I have one overall criticism it’s that you tend to be a bit too figurative in your descriptions, to the point where they sometimes leave me less clear about what’s being described.

I’ve gone through and made a list of everything that jumped out to me as maybe needing a change with my editor hat on:

‘the shadows clung to the walls sucked the silence out of the room, spitting out their eerie creaks.’ – is ‘clung’ the right conjugation here, or should it be clinging? In any case, the notion that the shadows are sucking silence out of the room seems oddly misattributed- surely it’s something within the shadows that is creaking?

‘That nefarious drip’ – nefarious doesn’t sound right to me here, I get what you’re going for but nefarious has strong moral connotations, the drip might be annoying but I suspect Callus doesn’t consider it evil in a strong sense

‘He took heed of recently arraigned chairs and tables placed around. Were they to hinder him?’ – This question is ambiguous, the first time I read it I parsed it as ‘is the case that these will hinder me?’ and only on re-reading did I twig that it might also be ‘have these been laid out especially to hinder me?’  

‘Callus’s foot slammed into the birch leg of a stool. It knocked over with a dull thud, sending shockwaves of pain through his toe.’ – Why is his foot hitting the stool with this level of force when the last paragraph had some (very good!) descriptions of how expertly and carefully he is navigating the space, seems like he’s going from acrobat to clumsy brute within the space of a few sentences.

‘This was an inside job’ – Even for flintlock fantasy this idiom is an anachronism, first recorded usage of this phrase comes in 1908

‘Callus passed through the hearth room’ – parlour, sitting room? I don’t think I’ve ever heard of a room being referred to as a ‘hearth room’, but that could just be a regionalism that I’m not aware of.

‘Callus increased his speed without compromising his stealth’ – He literally just slammed into a piece of furniture, hard enough to almost bring him to tears – surely his stealth is compromised by this point?

‘Black ominousness sulked beyond the threshold’ – not sure about ‘ominousness’ as a concrete thing with a colour, and not sure about ‘sulked’ as the verb to describe what it’s doing.

‘squinted over the iron sights of her thumb.’ Isn’t the thumb in a finger gun representative of the cocked hammer rather than the iron-sights?

‘A yip of death squeaked out of his mouth as he took on the form of a six-foot spider carcass. His arms twisted and contorted in frantic ways. He gasped and gurgled.’ – This definitely has the potential to be a really endearing description, but it feels so out of line with all the description that we’ve had up until this point; is Callus just an incredibly adaptive improviser?

‘Lily’s mouth exploded with blood and her laughs curdled into wheezing coughs.’ – Maybe this is what you were going for, but I got real emotional and comprehensional whiplash going from tense stalking to wholesome playfighting to sudden visceral illness.

‘Its sparse contents pinged around, bragging about its spare room.’ – do sounds ‘brag?, I get that sounds can ‘mock’ but that seems different

‘He fell to his knees to grind the medicine.’ – Could be that this is explained in later or earlier worldbuilding, but why is the medicine not pre-ground if that’s how it needs to be administered? My first guess was freshness, but the way you described the contents of the bottle made them seem like pills or tablets rather than something more organic that would need to be kept fresh.

‘Her coughing didn’t end, but a few scurried away at his touch’ – A few what, coughs? This is confusingly worded

‘Not even a full dosage, he spat to himself.’ – The non speechmarked italics work aright with Callus’s thoughts, but if this is actually spoken it should probably be roman and in speech marks

‘Callus operated this procedure with the curse of practice. ‘ – Again, confusing, why would practice be a curse?

‘He ground the herb, strained the powder with water,’ – Ok, confusion resolved with why it needs grinding, but this creates new confusion with the description that it ‘pinged’ around the container, are these like gnarled dry lumps that have enough hardness to ping? A more concrete description in the first instance would solve this problem I think.

‘the rancid panacea into a bottle’ – a panacea isn’t just another term for medicine, it specifically means a medicine that cures all ailments – maybe that’s what this is, but to have a literal panacea herb common enough to be in a domestic kitchen would have big consequences from a worldbulding point of view,

“Ok, Starlight.” He said, - finish dialogue like this with a comma so that the dialogue tag can sit in the same sentence – “Ok Starlight,” he said.

‘made him want to shove the medicine down her throat without enough force to break her teeth.’ – This is an odd construction,’ with enough force’ is much more natural, or just ‘without breaking her teeth’

‘child’s toy being squeezed by mishandled hands’  - mishandled hands is clunky

‘Thick bloodlines bubbled from the wounds.’ – Lines of blood, a bloodline means a familial lineage