Why such a difference between these two sisters?

First and foremost it can be genetics, just because they’re in the same pack does not really mean they came from the same mother. Many other small issues can be to blame such as: ph, nutrients, flushing the plant every 3 feeds, measuring your plants drained water (PPM) to ensure they’re flushed of all nutrients.

No 2 seeds will grow the exact same way as each other even if they were taken from the same plant. If you want identical plants then look into clones as they should grow very similar to each other.

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Seeds are not clones, Think of them like brothers and sisters, commonly referred to as its Phenotype, This is why their can be a lot of variance in different seeds. Same thing with humans. some siblings look a like and other not. All depends on the parents and luck of the draw. here's a small simple videos trying to explore it a bit about geno vs pheno, im sure you can google more cannabis related content. www.youtube.com/watch Breeders who care will refine their genetics to not have suck variation in the offspring but its not always possible, When profits are concerned corners are cut, as time=money and refining genetics takes time. If you grew them out and they practically had the same conditions and feed, its likely the seeds. Any seed can be susceptible to other mutations and stuff like any other living creature that can alter its growth. That might have happened a bit too, maybe. If you want the same results across the board look into growing seeds from ETHOS or other breeders who show stable genetics, they have some good standards in their seed stock. Or look at finding a mom, then cloning her out to have an identical population, this way if there is an error you know it will be more on how your treating it or how the environments shifting vs the plants genetics as they are all the same. Good Luck!

Honestly it could be both. Typically unless you are doing something vastly different to one plant vs the other they will grow roughly the same. So less likely your fault in its care, but you can’t rule it out entirely. There could have been a week or two where you watered one more and its roots were suffocated, the soil mix was slightly different ect ect.. just saying you can’t rule yourself out 100%. On the other hand, a lot of it is genetics. This is why pheno hunting is so important. You could sprout 100 seeds and have 5 clear winners that out performed any of the rest. Breeders to this all the time to get the best genetics. Likely your bigger plant is just genetically better at growing. I hope this helps.

Some of both likely. There is different phenom types that come from different plants.