Red [Scarlet Letter, warning sign, red wine, coals]
and blood [running from cleaved flesh, blush, anger's rush] come to mind.
The siblings and cousins things are puzzling.
Tallest by just a bit could refer to red light having the longest wavelength of visible light.
Except the "by just a bit" couldn't mean percentage - red/blue =~ 1.75 wavelength ratio - almost double.
Perhaps erythrocytes [red blood cells] with and without oxygen - two siblings.
Or the siblings could be the other two types of blood cells -- thrombocytes [platelets] and leukocytes [white blood cells].
Cousins could be other red cells. But that's more poetic than factual.
Sibblings: green and blue;
Cousins: red mixed with green and/or blue which means millions of sibblings
Off by a bit, might refer to the binary system. Though that is incorrect too because most probably a lot more bits don't match (there's just combination that's one bit off from full red: 255,0,0 vs 254,0,0