Here is the item again:
Whenever one of her six cats jumps on the kitchen
counter, (A)
Lorna just sighs. (B)
And reminds herself that everything tastes
better (C)
if there is a little cat hair in it.
You correctly realized that the period after sighs created a fragment. And reminds herself ... is a lonely verb phrase, so it cannot stand alone as a complete sentence.
To fix this fragment, you chose to do this:
Whenever one of her six cats jumps on the kitchen counter, Lorna just
sighs and reminds herself that everything
tastes better if there is a little cat hair in it.
You have correctly employed Punctuation Rule 8:
Main clause + Ø + lonely verb.
Keep us whistling!