[误] Kevin is the most warm-hearted of his brothers.

[正] Kevin is the most warm-hearted of all the sons in the family. /Kevin is more warm-hearted than any of his brothers.

注：“his brothers”并不包括凯文本人，所以用最高级不正确。

参考例句：What about his brothers, is that a problem? 那他的兄弟几个呢?会成问题吗?