What section 80DD says
"80DD. (1) Where an assessee, being an individual or a Hindu undivided family, who is a resident in India, has, during the previous year,
(a) incurred any expenditure for the medical treatment (including nursing), training and rehabilitation of a dependant, being a person with disability; or
(b) paid or deposited any amount under a scheme framed in this behalf by the Life Insurance Corporation or any other insurer or the Administrator or the specified company subject to the conditions specified in sub-section (2) and approved by the Board in this behalf for the maintenance of a dependant, being a person with disability, the assessee shall, in accordance with and subject to the provisions of this section, be allowed a deduction of a sum of fifty thousand rupees from his gross total income in respect of the previous year:
The meaning of the word "dependant" is also defined in section 80DD under Explanation . The said Explanation states
Explanation.For the purposes of this section,
(a) .............
(b) dependant means
(i) in the case of an individual, the spouse, children, parents, brothers and sisters of the individual or any of them;
(ii)......
It is clear that section 80DD is applicable only if the person on whom the amount is spending is any one of following
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wife or husband
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children
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parents
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brother
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sister
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wife's or husband's brother or sister
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parents' brother or sister