In a human relationship, these terms refer to a sibling, having at least one parent in common. Within Christian communities the same terms are used for a fellow believer in Christ, inferring the common bonding through the adoption into the family of God (Eph 1:5).
For example, Ananias referred to the newly converted Saul (later known as Paul), as “Brother Saul…” (Act 9:17). Likewise, a brotherhood or sisterhood is a community or association of people linked in comradeship by a common interest or religion.
Jesus said whoever does God’s will is His brother or sister, and He would not be ashamed to call them ‘brothers’, a term of warm affection (Mt 12:50; Heb 2:11). This is because, through emulating the character and lifestyle of Jesus, we will be conformed to His image. Christ is the First among many ‘sons' (i.e. people) redeemed by His blood into the eternal family of God (Rom 8:29).
We should love our Christian brothers and sisters deeply and genuinely forgive them, even to the extent of foregoing our wants, sacrificing for their benefit (1 Jn 2:9-11, 3:10, 3:16,17, 4:19-21). Be assured that whatever we do for ‘the least of God’s children’, we are [in fact] doing for Him (Mt 25:40). The Bible instructs us to not be the reason for our fellow Christians to stumble so it is of utmost importance to always act with integrity as our testimony and character can be quickly be tarnished through an unwise action of compromise (Rom 14:13,21; 1 Cor 8:13; 1 Tim 5:1,2). All our interaction with others must be wholesome. So that no one can accuse you of anything inappropriate, endeavour not to be in questionable situations or form any soul-tie alliances.
Care, respect and goodwill should be continually increasing and maintained through the challenges that any meaningful relationship brings. “Be devoted to one another in brotherly love. Honour one another above yourselves” (Rom 12:10; 1 Thes 4:9; Heb 13:1; 2 Pet 1:7).
Is my relationship with my natural and spiritual family God honouring?
mentions throughout his epistles (1 Sam 18:1-3, 20:17; Col 4:9; Phm 1:1,16). He was genuinely desirous of their spiritual growth as he constantly remembered them in prayer (Gal 4:19; 2 Tim 1:3). This was not motivated by self-interest, rather he wanted them to see and know the power of God in their lives too. While we will not agree on everything, there should not be intense dislike or ill feeling. Mercy and grace need to be exercised as we are charged with “keeping the unity of the Spirit…” (Ps 133:1; Eph 4:3). They are not perfect (actually I’m not either!).
Relationships, beginning initially with family members, are where we develop interpersonal ties that are vital for healthy social interaction throughout life. Paul also encouraged the Galatians to “do good to those who belong to the family of believers” (Gal 6:10).
Although the Bible is written from the masculine perspective and sometimes men just means 'men', generally the instruction is to both men and women unless the context implies otherwise.