。
She is very kind to us.
Literal
She [topic-は] us [to-に] very kind is.
親切 ('kind') is a na-adjective (always 親切な before nouns, 親切だ as predicate). The に here marks the recipient or beneficiary of kindness — 'kind to / toward someone.' Compare with 私たちにとって ('from our perspective / for us') in a sentence with a different nuance. The simple 親切です is everyday polite speech. Beyond personal kindness, 親切 also describes well-designed services or thoughtful explanations: 親切な説明 ('a clear / considerate explanation').