Objective: Information on risk levels is an essential part of the prevention of colorectal cancer (CRC). The objective of this article is to describe a tailored intervention carried out to inform the protagonists about the high risk of CRC due to family history and then to understand, through an interdisciplinary analysis, the mechanisms implemented during the intervention. Method: A randomized trial design was applied. The primary outcome was the colonoscopy uptake. The intervention concerned firstly the index patients with CRC or adenoma to advise and accompany them in the transmission of information about the elevated risk for their siblings. The siblings received tailored counselling carried out by a preventive nurse, by telephone then by mail. Epidemiological, linguistic and sociological cross-analyzes were carried out to understand the impact of the intervention. Results: Colonoscopy rate was 56.3% in the intervention group, 35.4% in the control group (P = 0.0027; 304 siblings). The linguistic analysis of recorded nurse’s interventions showed that, following the intervention (N = 59), the prevention themes were better assimilated while they remained vague and questioned in the discourse of the control siblings who had carried out the colonoscopy (N = 8). That was confirmed by the sociological analysis (four families) which also revealed the importance of information received from a health professional and of communication within families. Conclusion: The study showed the impact of transmission of information to convince siblings at high risk of CRC to carry out colonoscopy.