姐 - Jie3 - Older Sister, Miss

妹 - Mei4 - Younger Sister

姐妹 - Jie3 Mei4 - Sisters

If you’re going to be in Flushing tomorrow, watch out… The four 姐妹 will be coming through.

Update! There’s also 姊 - Jie3 - Older Sister. According to Jenny, 姐 is usually used to mean “Miss”, but for actual sisters, you use 姊.