first - im not a medical professional. this is all layman knowledge. when in doubt concerning a medical issue talking to a professional is often the best place to start.
my son has always been at or above 95% for height, but anywhere from 8-75% for weight, gaining over time. as the low weight was a concern for a time, we always had growth charts handy.
here is one i access commonly. it requires adobe acrobat, and it is sometimes slow to load. give it a few minutes if it is slow.
http://www.cdc.gov/nchs/data/nhanes/...l/cj41l017.pdf
at 3 years a child at just over 38 lbs is considered at the 95% for weight. this child is at 2 years and 12 lbs over this 3 year high end mark, and 16 lbs over the 2 year 95% mark.
so one question is what is the childs height %? obviously a taller child with a higher weight % is not as much a concern as a shorter child with the same %.
what is the childs diet and exercise routine & daily activities? if the child is eating reasonably and is active it may not be such an issue. having obsese parents might not mean poor eating and a sedentary day, but it is a reasonable concern.
my concern would be if the child is not active the risk for health problems is greater, such as type 2 diabetes, hypertension and some orthopedic issues, not to mention the social stigmas.
id be especially concerned about the diabetes if one of the parents has it. its known that childhood diabetes is linked to higher morbidity and mortality, just as it is for adults.
you likely knew at least one of parents, as you said nephew (unless by marriage)... was the parent you know obese throughout the younger years or did the weight gain occur later in adult life?