。
She is Tom's older sister.
Literal
She [topic-は] Tom [genitive-の] older-sister is.
お姉さん politely names someone else's older sister with the お prefix and ~さん suffix layered on. Japanese has separate words for older and younger siblings (兄/姉 vs 弟/妹), so English 'sister' splits along age lines — a lexical reflection of how seniority structures family. Talking about your own older sister you'd usually say 姉 or 姉さん in casual speech, dropping the polite trim that you'd keep for someone else's family.