Despite the unfair treatment in their youth, the siblings never held it against each other.

held 是动词 hold 的过去式，hold 的基本意思是"握住、持有"。 against 是介词，表示"反对、对抗"的意思。 held against 是一个固定短语，字面意思是"握住对抗"，引申为"因某事而怨恨某人"，指因为过去发生的事情而对某人怀有负面情绪。