Background: Diabetes mellitus (DM) is one of the serious non communicable diseases worldwide. Presence of DM patient in a family may be considered as risk factor for other family members to acquire the disease, due to DM inheritance nature and/or similar life style pattern among family members. This paper assessed awareness of DM patients' family members (DMPFMs) about DM occurrence and prevention. A cross sectional study was conducted in 2014 in two suburban cities of Ethiopia, namely Kemisse, and Kombolcha using an interviewer administered questionnaire among primary or secondary degree DMPFMs and controls. Based on eligibility criteria study participants were selected by health extension workers on house to house visit. Data were analyzed using SPSS version 20, and P value less than 0.05 considered as statistically significant. Results: Of the total 347 study participants, 45.5% (n = 158) had DMPFMs. Majority, 60.8% of DMPFMs and 73.0% of controls were males. Mean age of DMPFMs (30.06 years) was less than that of the controls (37.38 years). On living style, 51.9% DMPFMs, and 42.8% of controls were single. In both study groups, the majority of study participants attended grade 7-12. The likelihood of having good level of knowledge among DMPFMs were 2.94 times (AOR = 2.94 95% CI 1.87-4.86) higher compared to those who did not. Those attaining higher educational levels were 3.41 times (AOR = 3.41, 95% CI 1.31-8.91) more likely to have good level of knowledge, as compared to those who were unable to read and write. The likelihood of having good level of positive practice among DMPFMs were 3.38 times (AOR = 3.38% CI 2.05-5.58) higher as compared to controls. Participants who were living in Kombolcha were 2.33 times (AOR = 2.33 95% CI 1.31-4.12) more likely to have good level of practice, as compared to individuals from Kemisse. Conclusions: Diabetes mellitus family members in the Ethiopian suburban cities Kemisse, and Kombolcha had better knowledge and practice about DM compared to controls. But, the overall awareness about DM occurrence and prevention was relatively low. Thus, DM awareness campaigns should be strongly pursued regardless of family history and educational background to prevent further increase of DM in Ethiopia.