Though it is a leading cause of death, its etiology is complex and remains largely unknown so assumptions are things like sleeping in a dangerous position, a general failure during the critical development period, or an unknown underlying biological vulnerability.
A cohort study consisted of Danish infants in Denmark (doi:10.1001/jamanetworkopen.2022.52724) between January 1, 1978, and December 31, 2016, including siblings of children who died of SIDS. Siblings were followed up from the date of SIDS, date of birth, or immigration, whichever came first, and until age 1 year, emigration, developing SIDS, death, or study end. The median follow-up was 1 year.
In total there were 2,666,834 births of which 48 percent were female. By cross-linking the Civil Registration System with the Cause of Death Register, they found 1,540 infants who died of SIDS and there were 2,384 siblings. Among siblings, a higher rate of SIDS was found than the general population. The numbers are small but in relative terms having a sibling who died of SIDS was associated with a 4-fold higher risk of SIDS compared with the general population.
As an observational study, it can't tell clinicians if that higher risk is due to shared genetic or environmental factors but the authors recommend family history should be considered when assessing risk.