Bottom line
When you are ordered to or kept on active duty for more than 30 days, your dependents become eligible for medical care, either in a military treatment facility (MTF) or through TRICARE. To make that real, they have to be enrolled in DEERS. That enrollment is done with a DD Form 1172, and your unit commander is responsible for making sure it gets completed. If your dependents are not properly enrolled, care can be denied and a legitimate TRICARE claim can go unpaid.
What it means for you
The benefit is tied to the length of your orders, not just the fact that you have orders. The trigger is active duty over 30 days. Once you cross that line, your family qualifies, but the qualification only counts if they are in DEERS. Do not assume the paperwork is handled for you. Confirm with your S-1 that a DD Form 1172 was completed and that your dependents are enrolled in DEERS before you need the care, not after a claim gets denied.