Sister - Vine's Expository Dictionary of New Testament Words
Sister
[ 1,,G79, adelphe ]is used
(a) of natural relationship, e.g., Matthew 19:29; of the sisters" of Christ, the children of Joseph and Mary after the virgin birth of Christ, e.g., Matthew 13:56;
(b) of "spiritual kinship" with Christ, an affinity marked by the fulfillment of the will of the Father, Matthew 12:50; Mark 3:35; of spiritual relationship based upon faith in Christ, Romans 16:1; 1 Corinthians 7:15; 1 Corinthians 9:5, AV and RV marg.; James 2:15; Philemon 1:2, RV.
Note: In Colossians 4:10, AV, anepsios (cp. Lat., nepos, whence Eng., "nephew"), "a cousin" (so, RV), is translated "sister's son." See COUSIN.