Your two possibilities to claim a dependent are as follows:

You may be able to claim your nieces as Qualifying Child dependents if:

• The child must be related to you. The child can be your son, daughter, stepchild, eligible foster child, brother, sister, half-brother, half-sister, stepbrother, stepsister, adopted child or an offspring of any of them.
• Your child must be under age 19 or, if a full-time student, under age 24. There is no age limit if your child is permanently and totally disabled.
• Your child must live with you for more than half the year, but several exceptions apply (ie. away at school).
• The child cannot provide more than half of his/her support.
• You must be the only person claiming the child
• The child must be a US citizen, resident alien, national, or resident of Mexico or Canada.
• The child cannot file a joint return with his or her spouse.

You may be able to claim your sister as a Qualifying Relative dependent if:

• You provided more than half of her support in 2016
• She earned less than $4,050 in gross taxable income. (Social Security income generally doesn’t count here.)
•  She lives with you 365 days in the year or is related to you.
•  She isn’t a dependent on someone else’s tax return.
• She isn’t filing a tax return with a spouse (married filing jointly).
• She  is a U.S. citizen, resident alien, national, or a Canadian or Mexican resident

You can’t claim a dependent if you are a dependent on someone else’s taxes

See Rules For Claiming Dependents

Note: If your nieces don't qualify as Qualified Children, they may qualify as Qualified Relatives.