Would it be security risk to not upgrade to Sierra?
El Capitan is still currently supported by Apple. In fact, the most recent security update for El Capitan was released on 15 May 2017. For further information, refer to About the security content of macOS Sierra 10.12.5, Security Update 2017-002 El Capitan, and Security Update 2017-002 Yosemite.

You may also be interested in the following resources:


Contact Apple About Security Issues
Apple security updates


At some point Apple will cease to provide security patches to 10.11 and you would be arguably far less secure in specific. In general, you need to look at what security means to you and weigh the time to upgrade with the risk and time to reinstall from a backup.