Because your daughter has claimed the niece as a dependent, it's her responsibility to report it on her return.
from IRS.
A6. For purposes of the Premium Tax Credit, your “family” consists of yourself, your spouse if filing jointly, and all other individuals whom you claim as dependents. Your “family size” is the number of individuals in your “family.
Household income. For purposes of the PTC, household income is the modified adjusted gross income (modified AGI) of you and your spouse (if filing a joint return) (see Line 2a, later) plus the modified AGI of each individual whom you claim as a dependent and who is required to file an income tax return
