My 12 y/o sister is 68lbs and 5'. is her low bmi causing late puberty? (i had the same issue, high metabolisms run in the family).

Need to check BP : i do not know what the Q is but yes your sister weight affect not only her puberty but also other systemic serious effect. however high blood pressure do not go with low bmi and should be investigate the cause by her PMD. Here, we should rule out eating disorder which has its own criteria to diagnose. in general your sister need a careful follow up with her PMD.

Delayed puberty is : lack of testicular enlargement in boys by 14 & lack of breast enlargement in girls by 13, so she needs to be patient. If 3 -generation family members are tall, skinny & hypertensive since childhood, your sister's doctor will refer her to a pediatric cardiologist, pediatric ophthalmologist, geneticist & possibly an orthopedist to rule out Marfan Syndrome & other connective tissue disorders.