welcome to health care magic forum,
is a genetic disease.
The inheritance pattern of Noonan syndrome is autosomal dominant.
If one parents has the disease, and other is normal, then, Sons and daughters of this parent have a 50 per cent risk of inheriting the condition.
Children who do not inherit the mutated gene do not have the disorder and do not pass it down. So children of your boyfriend's sister have 50% risk. But your baby has no risk.