If a mum or dad of your proband is known to hold the pathogenic variant determined in the proband, the danger to the sibs of inheriting the pathogenic variant is 50%. As a result of reduced penetrance, sibs who inherit the pathogenic variant might or might not develop aHUS (see https://industrialchillerhire38158.get-blogging.com/33359273/facts-about-ahu-s-revealed