Two Year Old is underweight?

My sis's two year old was weighed today. She is 2 years and 4 months old, and weighs just 22 lbs. To put this in perspective, she has two cousins. Her 10 month old cousin weighs 19 lbs, and is considered a healthy weight. Her 7 month old cousin weighs in at 17 lbs. This kid eats like crazy. Her mom makes sure she eats at least 5 meals a day, per doctors orders. She has her on Pediasure. She drinks whole milk. Her mom makes sure she's eating good, balanced diet, so it's not like she's being raised vegan. Both her pediatrician and WIC doctor have expressed concern over the child's weight. She's otherwise healthy, except for ear infections, and a recent bout of strep throat. However, she has had three recent medical scares. She has been taken to the ER twice when her temperature plummeted, to the point that the doc in the ER pronounced her hypo thermic. She was in a heated house, wearing pajamas when her mother found her in her crib shivering. This happened a second time, her temp dropping to 94 before we got her to the ER. The most recent was finding blood in her stool. the doc didn't really give them any answers, except to say it was nothing to worry about. One person suggested she may have tapeworm. Any ideas on what's causing her symptoms and lack of weight gain?