Here’s a recent review of the topic. The fact that the review appeared in the Orphanet Journal of Rare Diseases is a little misleading, as familial dilated cardiomyopathy may be the most common cause of DCM and is an underappreciated condition. Screening of family members of patients who have “idiopathic” DCM is controversial. While this review recommends ECG and echocardiographic screening of first degree relatives of patients with DCM the 2005 ACC practice guidelines for chronic heart failure come short of such a recommendation, merely stating that such screening should be considered.
Genetic counseling and screening is complex and is discussed in references I linked to here.