Thanks for your question. I will try to help.
If your parents naturalised as UK citizens after being in the UK for a period of time, then unless they applied to register your sister as a UK citizen before your sister became an adult then she would not qualify for UK citizenship.
Any "indefinite leave to remain" here that she would have obtained when she was younger would also have lapsed if she has spent significant time (eg two years) outside the UK. However if she did obtain "indefinite leave to remain" by having it stamped her passport then she might - outside chance - be able to apply for a returning residents visa provided that she could show that she has strong family ties to the UK and lived most of her life in the UK:
Other than that, she does not really have any other right to enter the UK in addition to any other foreign national. IN these circumstances probably the best thing that she could do would be to apply for a long term family visai visa on either a 2, 5 or 10 year basis. This would allow her to enter/leave/enter the UK as much as she liked during the visa period but subject to her not spendnig more than 6 months in any 12 month period in the UK.
I am sorry that I could not have better news for you.
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