。
She writes to her son from time to time.
Literal
She [topic-は] sometimes son [to-に] letter [object-を] writes.
Classic ditransitive frame: subject (彼女) marks the writer, indirect object (息子) marked with に as the recipient, direct object (手紙) marked with を. Japanese ditransitives consistently use に for recipients of giving/sending/showing/teaching verbs — 友達にプレゼントをあげる ('give a friend a present'), 学生に日本語を教える ('teach students Japanese'), 母に電話をする ('call Mom'). Letters as a form of family communication carry a slightly nostalgic flavor — common practice for older generations and for spatial separation (like a child at university or working far away).