They aren't biological sisters, but their bond is deeper than that of blood sisters.
Tips
usage
亲 (qīn) here marks biological/full kinship — distinguishing 亲姐妹 from 表姐妹 (cousins) or 干姐妹 (sworn/non-blood sisters). The same prefix works in 亲兄弟 (full brothers), 亲生 (biological).