One very common way to establish that a male (or increasingly frequently female) character is a Nice Guy (even if the niceness is hidden under a gruff exterior) is to give them an instinctive desire to be a respectable big sibling to smaller or younger characters (in more cases female than male). These other characters may be truly younger siblings, or they may be completely unrelated, or even older, but the feeling is the same: to act as their protector.
The Big Brother Instinct serves to explain fan attraction to Moe tropes and devices as an emotional, rather than sexual, response. There seems to be something inherently comforting to an awkward male about being looked up to by a younger girl for support. Therefore, Moe can be wish fulfillment about taking a "big brother role" instead of about being a Love Interest. Rather amusingly, it's also quite often that the trope gets inverted, and the younger sibling wants to protect the older instead. Hilarity Ensues when both versions happen at the same time.
If both characters are male, the older (or in the case of twins, the physically stronger) brother will usually act as the younger/smaller brother's protector, to the point of treating him like a child, even when the younger brother is fully capable of protecting himself. This behavior can come across as loving and nurturing or as condescending and smothering. If the older brother goes to extreme, morally questionable lengths for the sake of the younger sibling, then they are a Knight Templar Big Brother.
Compare Mama Bear, Papa Wolf, Family Man, Extremely Protective Child, My Sister Is Off-Limits, Promotion to Parent. Compare/Contrast Big Brother Bully. While the two may seem mutually exclusive at first glance, they frequently overlap in both fiction and reality: the same sibling who makes their little sister's life a constant parade of pranks may have a If You Ever Do Anything to Hurt Her... conversation with her new boyfriend. Also, contrast with Disappointing Older Sibling when the character either lacks the Big Brother Instinct or is completely ineffectual at protecting the younger sibling.
Has nothing to do with the TV show Big Brother. The 1984 meaning of "Big Brother" is covered under Big Brother Is Watching.
