Thank you for your question.
If she is over the ages of 18 then you would need to apply under the adult dependent section. The form required will be the VAF4A form and both the guidance and the form can be found at the following link:
As mentioned earlier the threshold you would need to satisfy is as follows:
The applicant or, if the applicant and their partner are the sponsor's parents or grandparents, the applicant's partner, must as a result of age, illness or disability require long-term personal care to perform everyday tasks.
The applicant or, if the applicant and their partner are the sponsor's parents or grandparents, the applicant's partner, must be unable, even with the practical and financial help of the sponsor, to obtain the required level of care in the country where they are living, because-
(a) it is not available and there is no person in that country who can reasonably provide it; or
(b) it is not affordable.
You should provide evidence from her father or relatives in Pakistan regarding her situation in form of affidavits, bank statements of relatives, photographs of her sisters and brother forming an independent life and being married, her general health, statements from the person she is under the care of and the reason she is not able to be maintained by that individual, evidence that she does not have any bank accounts etc in Pakistan. Evidence of the fathers medical condition and age and his inability to look after your sister in law.
You do not need to satisfy any income threshold of £18,600 there is no requirement. Having said that you would need to provide evidence of income to state that once she will arrive she will not be a burden on the state so bank statements and other proof of income will need to be provided by yourself and your Mrs if you are jointly sponsoring her to the UK.
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