It's grammatically correct and it's not a natural way to speak but only because most people do not think about math that much when they speak. Good job

You are twice as strong as the others so the others grabbed half as many apples as you did from the apple trees. I wouldn't say it's exactly natural because I can't imagine a situation in which anyone would say that. It seems like a primary school maths problem "Bathtub A contains 3 times more water than bathtub B which looses water twice as fast as bathtub A..." You see what I mean?

It's technically correct but not natural. I would say "You are twice as strong as the others so they only picked half as many apples as you did from the trees." Use pick to replace grab to more clearly draw the picture of someone picking fruit and remove/ replace any redundant/ unnecessary words