The kanji 弟 originally depicted a bound string, representing things that are in 'order' or 'sequence'. A younger brother is someone who comes after, in sequence.
'Tei' — imagine a younger brother always getting 'TIED' to his older sibling's plans and activities.
The top part resembles 弓 (bow). Below it, strokes suggest a series of knots or a younger person following closely, as if tied to the 'bowstring' of their family.