Is it permissible to pay zakat to cover the expenses of my deceased sister's daughters, knowing that their father only pays 400 Egyptian pounds per month, and the expenses far exceed that?

What was spent previously does not count as zakat, due to the absence of the intention of zakat at that time. As for the future, it is permissible to pay zakat to the daughters of the sister if their father does not fully provide for them, provided they are eligible to receive zakat, since the uncle is not obligated to spend on his sister's daughters.