Probably not. It's complicated. They needed to have lived with you for more than half the year. There are two types of dependents,
"Qualifying Children"(QC) and standard ("Qualifying
Relative" in IRS parlance even though they don't have to actually be
related). There is no income limit for a QC but there is an age limit, a
relationship test and residence test. Only a QC qualifies a taxpayer for the
Earned Income Credit and the Child Tax Credit. They are interrelated but the
rules are different for each.
Since they did not live with you for more than half the year, they are the QC of the taxpayer they did live with. That is either your deceased sister or your father. If neither of them are required to file a tax return, or they can get no benefit from claiming the children and you supported the children, you can claim them as a qualifying relative but not a QC (no child tax credit or EIC for 2017).
.A child closely related to a taxpayer can be a
“Qualifying Child (QC)” dependent, regardless of the child's income, if:
1.
He is under age 19, or under 24 if a full time
student for at least 5 months of the year, or
is totally & permanently disabled
2.
He did not provide more than 1/2 his own
support
3.
He lived with the relative (including temporary
absences) for more than half the year
4.
He is younger than the relative (not applicable
for a disabled child)
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 (this
essentially means that you have the parent’s permission to claim the child, if the child also lived with the
parent more than half the year)
6.
If the parents of a
child can claim the child as a qualifying child but no parent so claims the
child, no one else can claim the child as a qualifying child unless that
person's adjusted gross income (AGI) is higher than the highest AGI of any of the
child's parents who can claim the child.
A person can still be a Qualifying
relative dependent, if not a Qualifying Child, if he meets the 6 tests for
claiming a dependent:
1. Closely Related OR live
with the taxpayer ALL year
2. His/her gross taxable income for the year must be less than $4,050 (2016-17)
3. The taxpayer must have
provided more than 1/2 his support*
In either case:
4. He must be a US citizen or resident of the US, Canada
or Mexico
5. He must not file a joint
return with his spouse or be claiming a dependent of his own
6. He must not be the
qualifying child of another taxpayer
https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...
*If
no one person (or married couple) provides 50% of the support (for example your
brother was also sending support and your father provided housing), then a "multiple support agreement”
(IRS Form 2120) can be used, to allow you to claim the dependent. https://www.irs.gov/pub/irs-pdf/f2120.pdf
