。
She's in the basketball club.
Literal
She [topic-は] basketball-club [in-に] is-belonging.
部 ('club') is the standard suffix for school clubs and circles — テニス部 ('tennis club'), 写真部 ('photography club'), 演劇部 ('drama club'), バスケット部 ('basketball club'). The Japanese school-club system (部活動 or 部活 for short) is a deeply embedded part of school life from middle school through high school, with daily practices and tight social bonds; foreign students arriving in Japan are often surprised by how central club activities are to teenage social identity. ~に入っている ('be a member of, be in') uses the resultative ~ている — once you've joined a club, the membership state continues. バスケット is a casual abbreviation of バスケットボール.