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The chief mother isn't your own mother, but she is the chief of your mother. She's your mother's lady sibling, who has always owned your mom. She's the chief mother. She's your aunt.
The reading for the 母 part is just the first は in はは, rendaku'd. You can also think of your aunt as a sheep, too, if you'd like.
I wonder why my aunt didn’t come to our wedding ceremony.