There are two types of dependents--qualifying child and qualifying relative. The credit in TurboTaxAshby's answer is for someone who can be claimed as a qualifying relative dependent. If your disabled sister it totally and permanently disabled as defined by the IRS, you may be able to claim her as a qualifying child dependent and get additional credits for her. Here is the definition of totally and permanently disabled:
Your child (or sister in your case) is permanently and totally disabled if both of the following apply:
1. He or she cannot engage in any substantial gainful activity because of a physical or mental condition.
2. A doctor determines the condition has lasted or can be expected to last continuously for at least a year or can lead to death.
Here are the requirements to be claimed as a qualifying child dependent:
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, (b) under age 24 at the end of the year and a full-time student, or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you (or your spouse, if filing jointly) unless disabled.
If she is not totally and permanently disabled and/or doesn't meet the requirements to be claimed as a qualifying child dependent, she may qualify to be claimed as a qualifying relative dependent if she meets these requirements:
1. The person cannot be your qualifying child or the qualifying child of any other taxpayer. A child is not the qualifying child of any other taxpayer if the child's parent (or any other person for whom the child is defined as a qualifying child) is not required to file an income tax return or files an income tax return only to get a refund on income tax withheld.
2. The person either (a) must be related to you or (b) must live with you all year as a member of your household.
3. The person's gross taxable income for the year must be less than $4,150 in 2018.
4. You must provide more than half of the person's total support for the year.
5. The person must be a U.S. citizen, U.S. resident alien, U.S. national, or a resident of Canada or Mexico.
