Question

She is cooking for my sister's children in present simple
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Answer 1

she cooks for my sister's children

Step-by-step explanation:

present simple is when something happens regularly

this sentence is in present-continuous tense in which something is a temporary activity

if the subject has pronouns, to convert present-continuous to present simple we add s to the verb

Answer 2

She cooks for my sister's children.

Step-by-step explanation:

cooks is the present simple if the subject has pronouns