poor relation idiom
（同类中）略逊一筹的事物（或人）；不受青睐的事物（或人）
something or someone similar to but less important than another thing or person, and that people do not consider equally valuable
The air force and navy were modernized but the army, very much the poor relation, was not. 空军和海军都已实现了现代化，而陆军却相形见绌。