Welcome to the Apple Support Communities. I see from your post that you're wondering why you're not being prompted to update to iOS 11 on your iPhone 6s, when the rest of your family already has that version. I know how important it is to keep your iOS software up-to-date, and I am happy to help.
- Could you possibly have an iOS Beta Profile on your iPhone? To check, please go to Settings > General > Profiles & Device Management. If you find a Beta Profile installed there, delete it.
- Then, Restart your iPhone, iPad, or iPod touch.
- Next, it's always a good idea to do a backup before an update: How to back up your iPhone, iPad, and iPod touch
- Finally, go to Settings > General > Software Updates and see if your update is available.