brothers; older and younger brothers
Formal or literary term specifically for brothers (male siblings), as opposed to the common きょうだい which can refer to siblings of any gender.
The affection between the brothers was deep.
兄弟の絆は強い。
The bond between brothers is strong.
second cousin
The kanji 兄 (elder brother) and 弟 (younger brother) combine to form 兄弟. The reading けいてい uses the on'yomi of both characters, giving it a formal, literary feel compared to the kun'yomi-based きょうだい.