What is "uterine sister"

sibling \sib"ling\ [sub + -ling.] (s[i^]b"l[i^]ng), n. a brother or a sister.

Note: Siblings have at least one parent in common. Those related only by a common mother are uterine siblings; those related only by a common father are agnate siblings or consanguine siblings (a legal term). A sibling having both parents in common is a sibling-german or a full brother or full sister. These modifying terms are more commonly used for the more specific uterine brother, uterine sister, agnate brother, brother-german, etc.

n. A half sister having the same mother but a different father.