General

It's not solely about genetics. Even then, unless they are identical twins, they are not genetically identical.

But even if they were, weight is not decided by genetics alone. They might have different lifestyles, different circles of friends, and encounter different events during their lives which might influence their lifestyles.

(I recently explored this in my story "Riddle with sister" )

However, according to my experience, sisters more often have similar weights than not.

Most sisters I know, are either all thin, or all fat. I know very few exceptions where one is significantly fatter than the other.

Not everyone gets the same genes. There are families who might have two sisters with different hair color or eye color.

My sister is 5'3" and about 115 pounds. I'm 5'8" and 353 pounds! I'm obviously quite fat now, but even when I was thin I was much larger than her. I take after Dad in my build, she takes after Mom.

What happens is the bigger stronger one grabs the food first.