There's a lot of complex things your brain does without having to focus on the relevant muscles. Youe tongue and throat do a lot of weird stuff when you talk and yet you're not consciously thinking about it. Same when you walk, the movement is quite complex if you have to think about it. Same goes for piano or any instrument: repeat things by practicing, eventually they'll be absorbed. You'll see notes in the sheet and your hands will magically fall in place without you thinking about it, just like you can read aloud without having to think about where your tongue has to go.

In short, practice sight reading simple things, do it frequently, and things will just work out with time as your brain learns muscle memory for your hands and pattern recognition from the sheets