Thanks for your question.
If you are a UK citizen and they do not have any other connection to the UK then the only possible visa would be an "adult dependent relative" visa.
The .gov webpage on this is here under the heading "Coming to be cared for - you’re an adult dependent relative":
You willl see the eligibility criteria listed. You must be able to prove that they need care to do every day tasks. You must also be able to prove that the care they need is not available or affordable in the country where they leave (eg sudan if they return there).
You must also be able to show that you can support/accomodate them without the need to access public funds.
If the proper care is not available in Sudan then the best way forward would probably be for them to return there, evidence the lack of available care there for their condition and then apply.
It's a really complicated application. This means that you should really stronlgy consider instructing a UK based solicitor in order to draft the application and collate the necessarysupporting documentation.
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