Can a retired SM make his/her parents and in-laws their legal dependents?

SSG (Join to see), I reworded your question to what I believe you were asking. If this is not the case, let me know.

I assume you're asking in order to have them added as dependents for eligibility on TRICARE, have a USIP ID card, other benefits that a traditional dependant would have.

What you are asking is referred to as "Secondary Dependency". In the military there are four categories of individuals that can be declared a secondary dependent (stating in case others see your question when looking for answers).

● Parent, parent-in-law, step-parent, parent by adoption, or any person who stood ‘in-loco parentis’ (in the place of a parent) to you for at least five years prior to your emancipation.
● Unmarried children ages 21 and 22 who are enrolled in an accredited institution of higher education on a full-time basis.
● Ward of the court, unmarried, and placed in the permanent legal physical custody of the member, or if not permanent custody for a period of at least 12 consecutive months.
● Unmarried child over age 21 incapable of self-support because of mental or physical incapacity that occurred while the child was considered a dependent of yours as a member or retired member, or is considered the dependent of a deceased member (while under age 21 or under age 23 and a full-time student).

It usually surprises people (it did me when I went through the process) that approval is given by DFAS* and not at your local RAPIDS site (ID card office), although they will process the application once it's been approved.

One of the primary criteria is that you must show you financially support them by providing at least 50% of their living expenses (you WILL have to provide financial documentation proving it).

The process changes a bit from service to service so make sure you are consulting the right one* as some might have a few additional administrative requirements.
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* DFAS Secondary Dependency overview - https://www.dfas.mil/militarymembers/secondarydependency/sdc/
* Army Secondary Dependency process, forms, FAQs, etc. - https://www.dfas.mil/MilitaryMembers/SecondaryDependency/secondarydependency_Army/ (note: there are different pages addressing the Navy, Marines, and Air Force).

From a civilian perspective....it should be possible. You would probably need to consult a lawyer to see what legal steps are needed to be completed.

Parents or in laws can be dependents - yes. If they meet your service's criteria to become a dependent.

"Can I add my parents to my TRICARE?
If your parents and parents-in-law meet your Service's criteria to become your dependents, they may: Get care at a military hospital or clinic on a space available basis; Get their drugs at a military pharmacy; and. May enroll in TRICARE Plus (if available)."

https://tricare.mil/FAQs/General/GEN_Dependent_Parents#:~:text=If%20your%20parents%20and%20parents,TRICARE%20Plus%20(if%20available).

Here's TRICARE page about TRICARE in retirement: https://www.tricare.mil/LifeEvents/Retiring#:~:text=Family%20members%20who%20are%20eligible,remain%20eligible%20when%20you%20retire.

This might apply to parents or in laws: Are You Eligible for Medicare?
Family members who are eligible for Medicare must have Medicare Part B coverage to stay eligible for TRICARE when you retire. If you had Medicare while on active duty, you remain eligible when you retire.

If you're eligible for Medicare, you must have Medicare Part B coverage to stay eligible for TRICARE when you retire. >>Learn More

It doesn't seem to say who you can add after you retire but I feel like you should be able to add dependents even then if they meet the criteria to be a dependent.