This article is based on a qualitative empirical project about a distinct kinship group who were among the first identified internationally as having a genetic susceptibility to cancer (Lynch Syndrome). 50 were invited to participate (42 were tested; eight declined genetic testing). 15, who had all accepted testing, were interviewed. They form a unique case study. This study aimed to explore interviewees’ experiences of genetic testing and how these influenced their family relationships. A key finding was that participants framed the decision to be tested as ‘common sense’; the idea of choice around the decision was negated and replaced by a moral imperative to be tested. Those who did not follow ‘common sense’ were judged to be imprudent. Family members who declined testing were discussed negatively by participants. The article addresses what is ethically problematic about how test decliners were discussed and whether these ethical concerns extend to others who are offered genetic testing. Discussions showed that genetic testing was viewed as both an autonomous choice and a responsibility. Yet the apparent conflict between the right to autonomy and the moral imperative of responsibility allowed participants to defend test decliners’ decisions by expressing a preference for or defending choice over responsibility. The ‘right not to know’ seemed an important moral construct to help ethically manage unpopular decisions made by close family who declined testing. In light of this research, the erosion of the ‘right not to know’ in the genomic age could have subtle yet profound consequences for family relationships.