Who are “the least of these my brothers” in Matthew 25?

Synonyms permit remez linking. "Little" and "least" couple these verses:

   Mt 18:6 But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and [that] he were drowned in the depth of the sea.

   Mt 25:40 And the King shall answer and say unto them, Verily I say unto you, Inasmuch as ye have done [it] unto one of the least of these my brethren, ye have done [it] unto me.

As such, Mt 18 defines them as the little ones "which believe in me". SO the brethren in Mt 25 are believers.