I-800 Orphan adoption theoretical question

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My (now) wife recently came over from South Africa to the US on her K1 and we are now adjusting her status. Weeks after we got married, her mother passed away unexpectedly. She still has her dad who is in not the greatest health and her 11 year old sister. I'm trying to understand what options would be available if her dad passes away in the next few years. I've read about the I-800 and it sounds like it is a 2 month process that has to be done before her sister's 16th birthday. However, would using the I-800 to adopt an orphan family member in this way be possible? Would it be practical? Would the entire process only take the couple months of processing the I-800 or would we have to adopt her through South Africa before even starting the I-800?

 

Thanks for any thoughts, I know this is a bit of a weird question as it is all theoretical at this point, but her family is redoing wills and life insurance at the moment so it is a good time to figure out plans for her sister if something did happen to her dad.