We found one dictionary that defines the word stepmother-in-law:

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1. stepmother-in-law: Wiktionary

Definitions from Wiktionary (stepmother-in-law) ▸ noun:  A stepmother of one's spouse. ▸ Words similar to stepmother-in-law ▸ Usage examples for stepmother-in-law ▸ Idioms related to stepmother-in-law ▸ Wikipedia articles (New!) ▸ Popular adjectives describing stepmother-in-law ▸ Words that often appear near stepmother-in-law ▸ Rhymes of stepmother-in-law ▸ Invented words related to stepmother-in-law