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You cannot claim a person as a dependent unless that person is your qualifying child or qualifying relative.
For qualifications - see page 12 -
Based on your information
- disabled 42-year-old son is taxpayer's qualifying child., and
- his disabled half-sister may be either his qualifying child or qualifying relative depending on requirements.
Tests To Be a Qualifying Child
1.The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2.The child must be (a) under age 19 at the end of the year and younger than you (or your spouse if filing jointly), (b) under age 24 at the end of the year, a student, and younger than you (or your spouse if filing jointly), or (c) any age if permanently and totally disabled.
3.The child must have lived with you for more than half of the year.
4.The child must not have provided more than half of his or her own support for the year.
5.The child must not be filing a joint return for the year (unless that joint return is filed only to claim a refund of withheld income tax or estimated tax paid).
Tests To Be a Qualifying Relative
1.The person cannot be your qualifying child or the qualifying child of any other taxpayer.
2.The person either (a) must be related to you in one of the ways listed under Relatives who do not have to live with you, or (b) must live with you all year as a member of your household (and your relationship must not violate local law).
3.The person's gross income for the year must be less than $3,950. ($4000 for 2015)
4.You must provide more than half of the person's total support for the year.