This is from the TRICARE Website: "Stepchildren
Stepchildren do not have to be adopted for them to be covered by TRICARE. As long as the sponsor and the parent of the stepchildren are married, the stepchildren are covered by TRICARE. If the marriage ends in divorce, the stepchildren lose eligibility on the date the divorce decree is final.
If the sponsor adopts his or her stepchildren, and the marriage ends in divorce, the adopted stepchildren remain eligible TRICARE up to age 21 (or 23rd if in college, see "College Students" below) even if the sponsor gets divorced or remarried." http://www.tricare.mil/mybenefit/Pr...IsEligible
I would contact TRICARE and speak to a Rep to hammer out all the details. Good Luck!