Sister Sizing is an alternate bra size. Simular but not the same

Bras: What is Sister Sizing? Bra Sizing Explained by Kimmay

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Sister Sizes: The Bra Secret Every Person Should Know – Sister Sizes & Equivalent Bra Size Chart – ThirdLove

Every bra size has a sister size: This is how it works

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What is Sister Sizing? – Fleur of England

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Q&A: Bra Sister Sizing Frequently Asked Questions - ParfaitLingerie.com - Blog