Complicated indeed!
Stepmother-in-law would work, though the ‘in-law’ part isn’t accurate.
If you wanted to be accurate you could go with stepmother-‘in-law’, stepmother-in-common-law, or even stepmother-in-life!
My partner’s stepmother is simplest. Or her name!
Sorry, got carried away - I like word puzzles.
Stepmother-in-law would work, though the ‘in-law’ part isn’t accurate.
If you wanted to be accurate you could go with stepmother-‘in-law’, stepmother-in-common-law, or even stepmother-in-life!
My partner’s stepmother is simplest. Or her name!
Sorry, got carried away - I like word puzzles.