The only person I know who has a skeleton in the cupboard is

不是插入语,I know 和who has a skeleton in the cupboard 都是定语从句
我所认识的唯一的在柜中藏骷髅的人便是乔治•卡尔顿,他甚至引以为自豪.
再问： 为什么who放在know后
再答： who和know没关系，分属两个定语从句，前面省略了that The only person that I know who has a skeleton in