Can I adopt my half sister?

I wish you had started sooner, as there is an immigration issue. If you have already legally adopted your half sister in Thailand, you won't have to readopt her in the U.S., once she comes home, unless you choose to do so. HOWEVER, your half sister will not be entitled to an adoption visa unless she is under age 16 when you file the "orphan petition" to begin the immigration process with the USCIS. If you can't file on time, she won't be able to get an adoption visa, and could have to wait a long time -- like years -- for a regular immigrant visa. You WILL be able to apply for an orphan visa for your sister, because you are a U.S. citizen and your spouse is too; at least one of the adoptive parents must be a U.S. citizen. You will have to have a homestudy by a qualified adoption professional, and submit that with some other documentation on an I-600 application to the USCIS. A homestudy can sometimes take a couple of months to complete, and the other paperwork that is needed for the USCIS approval can also take time to pull together. And the USCIS is likely to review your half sister's paperwork very carefully to make sure that it is legal and that she meets all of the requirements for an adoption visa. So if you don't get started immediately, you may wind up in a situation where you cannot get a visa for her in a timely manner. Sharon