The family design is at the heart of all of creation and it's this very design that Paul uses to instruct Timothy on how the church should operate. Because the church is a family to belong to, naturally we should treat each other as brothers and sisters, as we endeavour to live our lives in harmony under the Lordship of Christ.
After Paul dropped off Timothy to Ephesus in order to help shepherd the church, probably 8 years had passed since they had seen each other. So, Paul takes pen to paper, not knowing when they would reunite, and sees it necessary to write to Timothy concerning how the church should operate. At the centre of all this comes his first exhortation - preach and protect the gospel.