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 problem. And reminds herself ... is a lonely verb fragment, which 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!