I’m not talking about denominations. I’m talking about non-Catholics who have a genuine faith in Christ and seek to serve Him. Would you consider them brothers and sisters in Christ?
The difference is that the Catholic is a member of the Mystical Body of Christ. Christ invites these to become one with Him, one body, a spiritual society. In order to be a member he needs to accept all the graces that Christ makes available to perform his work. Baptism in the proper Church is the first expression of obedience of Faith. If the parts are to achieve a common end, then they would require no self imposed hindrances to their function.
From New Advent(Church)
"That analogy(Body of Christ, the Spouse of Christ, the Temple of God) expresses indeed the variety of function, the unity of directive principle, and the cooperation of the parts to a common end, which are found in a society; but it is insufficient to explain the terms in which St. Paul speaks of the union between Christ and His disciples. Each of them is a member of Christ (1 Corinthians 6:15); together they form the body of Christ (Ephesians 4:16); as a corporate unity they are simply termed Christ (1 Corinthians 12:12).
The intimacy of union here suggested is, however, justified, if we recall that the gifts and graces bestowed upon each disciple are graces merited by the Passion of Christ, and are destined to produce in him the likeness of Christ. The connection between Christ and himself is thus very different from the purely juridical relation binding the ruler of a natural society to the individuals belonging to it. The Apostle develops the relation between Christ and His members from various points of view. As a human body is organized, each joint and muscle having its own function, yet each contributing to the union of the complex whole, so too the Christian society is a body “compacted and firmly joined together by that which every part supplieth” (Ephesians 4:16), while all the parts depend on Christ their head. It is He Who has organized the body, assigning to each member his place in the Church, endowing each with the special graces necessary, and, above all, conferring on some of the members the graces in virtue of which they rule and guide the Church in His name (4:11). Strengthened by these graces, the mystical body, like a physical body, grows and increases. This growth is twofold. It takes place in the individual, inasmuch as each Christian gradually grows into the “perfect man”, into the image of Christ (Ephesians 4:13, 15; Romans 8:29). But there is also a growth in the whole body. As time goes on, the Church is to increase and multiply till it fills the earth. So intimate is the union between Christ and His members, that the Apostle speaks of the Church as the “fullness” (pleroma) of Christ (Ephesians 1:23; 4:13), as though apart from His members something were lacking to the head. He even speaks of it as Christ: “As all the members of the body whereas they are many, yet are one body, so also is Christ” (1 Corinthians 12:12). And to establish the reality of this union he refers it to the efficacious instrumentality of the Holy Eucharist: “We being many, are one bread, one body: for we all partake of that one bread” (1 Corinthians 10:17 — Greek text)".