Yes, but it will flag your sister's return to the IRS.
You can go ahead and claim the dependent care credit and advise your sister to file an amended return. She will have to wait until her original return is fully processed (she gets her refund) before amending.
In addition to income tax, she will have to pay social security and Medicare tax (in the form of "self employment tax" [SET]), unless the total amount you paid her was less than $2230. SET is about 14%. But the income is earned income and may qualify her for more earned income credit.
