Mom, dad and sis, but not bro

PALINDROMES

["Think of words that read the same backward as forward—this answer describes a group of them, minus one that doesn’t fit the pattern."]

The clue lists family members—"mom," "dad," and "sis"—which are all words that read the same backward (like "mom" spelled backward is still "mom"). The phrase "but not bro" hints that "bro" doesn’t fit this pattern, steering you toward words with mirrored spelling. That’s the definition of a palindrome, and since the answer is plural, it’s "palindromes."