They belonged to a family by the name of Northup, one of whom, removing to the State of New-York, settled at Hoosic, in Rensselaer county.
1. 你对句子不理解可能主要是因为插在句子中间的现在分词短语 removing to the State of New-York，如果去掉它，句子就变得简单了：
They belonged to a family (先行词) by the name of Northup, one of whom settled at Hoosic (定语从句), in Rensselaer county.
one of whom 中的 whom 是关系代词，引导非限制性定语从句，修饰先行词 family。
2.再回过头来看那个现在分词短语：removing to the State of New-York，它也是用作定语，修饰 whom。
句意：它们属于一个名为诺瑟普的家族，该家族中有一个人迁往了纽约州，后来在伦斯勒县的胡西克定居。
