do you consider this child abuse?

my step son and step daughter are very overweight. they both are about the same weight and height but the 8 year old boy is 4 ft 3 and 100lbs. his sister is 10 next month and same height and weight. she doesnt look as heavy as he does and it literally worries me. he has been diagnosed with exercise induced asthma and when hes at our house hes eating stir fries and plenty of fresh fruit and vegetables but when hes at his mothers, hes sitting in front of an xbox drinking COCA COLA and eating KFC and big macs... i know some kids are overweight because of "genes" or metabolic conditions but its pretty evident why theyre as big as they are. i just picked the step son up from school and hes got an ear infection, i went to give him panadol and he is having the dosage a 15 year old should have based on weight. my 8 year old daughter is 55lbs!