不同情况有不同使用法.
评论:sb is insincere to sb
对人:She's been insincere to him (been过去时,being进行时)
不用sth,insincere已经是"事情".再加sth就是原因了,表明为什么不是真心对待他.
She was insincere to him because he cheated on her.她之所以对他不真心,因为他偷过情.
我说的"对事"不是对事情,是别人对这件事评论...sb is (has been) insincere to sb...