Seeing how she's gone from "Flat to DD" I can tell you pretty assuredly that she already knows about the change - with that being the case it's reasonable to say that she's already seen/talked to a doctor about it if it is medically relevant.

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I know you've been getting pretty hostile with anyone in the thread who's told you to stop fixating on this (since it's kinda creepy) but you should really heed their advice and back off. Your cousin is 24, a grown adult, and can take care of herself. Even if her body growth is due to a medical complication it's still none of your business and If she wants your advice I'm sure she'd approach you about it. Until that time, it's probably best to stop obsessing about your adult cousin's breast development / augmentation...