At 18, she is no longer subject to guardianship as she is a legal adult. This is true in most jurisdictions but Texas is unique and may be different. Does she have any special needs, other than needing to be out of her home?
There are several terrific Texas attorneys who respond here periodically so perhaps they will have some state specific advice.
Use the AVVO.com web site to find an attorney in your area. In addition to that, contact your local bar association for referral to an attorney who specializes in this or talk to friends and neighbors to ask about an attorney they have used and liked. Often, but not always, the attorney will do an initial consultation free of charge. You will then be in a better position to determine what to do next. Best of luck to you!
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Eliz. C. A. Johnson
Post Office Box 8
Danville, California 94526-0008
Legal disclaimer: I do not practice law in any state but California. As such, any responses to posted inquiries, such as the one above, are limited to a general understanding of law in California and not to any other jurisdiction. In addition, no response to any posted inquiry should be deemed to constitute legal advice, nor to constitute the existence of an attorney/client or other contractual or fiduciary relationship, inasmuch as legal advice can only be provided in circumstances in which the attorney is able to ask questions of the person seeking legal advice and to thus gather appropriate information.
Unless your niece is otherwise incapacitated or disabled she is a legal adult. You cannot cover her on your insurance unless she meets special circumstances. I would check with your insurance company and see if there are any circumstances applicable int his case. A full time student although living with you is otherwise covered on his or her parents insurance so depending upon what the home circumstances are even if she is living with you she may be covered at home. Also universities frequently have some low cost health insurance and plans.