According to Oxford Learner Dictionaries, a word family has two definitions:
1) a group of related words that are formed from the same word a word family consisting of “help,” “helper,” and “helpful”
2) a group of words with particular features in common “Cat” and “hat” are in the same word family.
Take, for example, the word BELIEF.
A list of synonyms abounds: credence, credit, assurance, faith, trust, presumption, conviction and so on.
The Thesaurus then offers its antonym, DOUBT, with a range of related words and phrases: miscreance, infidelity, skepticism, suspicion, question, demur, dispute.
It is my belief that all writing begins (and often ends) with a mastery of words. Let us see this as an opportunity to explore the words and the way they work in our poems today, whether you choose the families of words that can be used as rhymes, derivatives of words which can be played with in many ways or a keyword, along with its synonyms and antonyms. The theme of the poem itself is of your own choosing, as is the length, form and style of your piece.