How do you say I have a younger sister and an older brother in Japanese?

To say "I have a younger sister and an older brother" in Japanese, you would say:

私は妹と兄がいます。

(Watashi wa imōto to ani ga imasu.)

妹 (imōto) means "younger sister" and 兄 (ani) means "older brother." がいます (ga imasu) is the polite way to say "I have."