There is nothing the UK can do to prevent you adopting them if Indian law, indeed, permits this (and that is by no means certain!), but if your sister is still alive then any notion of their subsequent settlement in the UK is completely misguided. It's not permitted for living parents to gift a child to relatives in the UK, and then expect that child to qualify for settlement in the UK under UK immigration laws.
SET7.18 How do adopted children qualify for settlement?
In the case of UK-based adopters hoping to adopt a child which has been given or 'gifted' to them by relatives abroad, there is very little chance of them being able to bring the child into the UK. The UK's international obligations prevent any acceptance of such practices. Even if a foreign adoption has been completed, the requirements of the Immigration Rules will prevent an entry clearance being issued to such a child.
http://www.ukba.homeoffice.gov.uk/polic ... /set/set7/