Yes, on the blood work! Forgot about hypothyroidism and I even have it.
I think it's genetic so it's completely plausible that all siblings could have it. A simple daily med straightens it out.
I agree with BC that thin doesn't automatically equal healthy. My daughter is an average weight for a 16 year old, but she's short, only 5'2. So she seems like she's carrying a few extra pounds when she's actually not (but try convincing her of that). She can stand next to her friends who are 5'4 and up and who weigh close to the same amount, but have different body types altogether because of their height. For her to be as "thin" as they are, she would have to weight less than 100 lbs. And because she's a dancer (8 years in) she has a lot of muscle, so for her to lose weight would actually be unhealthy in her case.