Can you force a family member to get genetic testing?

No. An individual must give permission to have genetic testing performed. Carrier testing for genetic disease is usually not done in children until they are able to give informed consent.
No special risk. This is an important question that is incredibly complex. It is not appropriate to "force" someone to have genetic testing, whether it is a question of diagnosis, risk factor assessment, inheritance patterns, or a myriad of other reasons. An ethically appropriate strategy is to provide a balanced, science-based, discussion of the reason for genetic testing, and then allow the person to decide.