This is my little sister Sakiko. She's exactly a year younger than me and she's currently studying for entrance exams.
Literal
Little-sister [genitive-の] Sakiko [copula-です]. Me [with-と] consecutive-year-child, now exam-student [copula-です].
Two notable vocabulary points. 年子 (としご) is a specific word for 'two siblings born within a year of each other' — a precise concept English needs a whole phrase for. 受験生 ('exam-student') refers specifically to students preparing for high school or college entrance exams — a culturally loaded status in Japan. 俺 is the rough masculine first-person; tells us the speaker is probably a young man.