Crossword Genius

Resist pests, horrible semi-siblings (11)

Ross

I believe the answer is:

stepsisters

I'm a little stuck... Click here to teach me more about this clue!

'semi-siblings' is the definition.
I can't tell whether this definition defines the answer.

'resist pests horrible' is the wordplay.
'horrible' is an anagram indicator.
'resist'+'pests'='resistpests'
'resistpests' anagrammed gives 'STEPSISTERS'.

Can you help me to learn more?

(Another definition for stepsisters that I've seen is " Relatives by marriage".)

I've seen this clue in The Guardian.
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