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).

Yes they can but it is under certain circumstances financial issues, medical dependency etc butthe bottom line is yes.

Yes, they can, My Mother's Aunt who served 30 years on active duty in the US Navy had that situation with Her own elderly Mother. She took care of housed, fed and looked after Her Mother Who was well into Her late 90s when She passed. The duty stations where She served Her elderly Mother whom We called Granny lived with Her.