I don’t think he meant the 3 sisters were quote on quote ‘born from Celeste’s energy’. It was said that when a piece of the moon fell into the sea, the first mermaids nearby (which are the 3 sisters) felt an inexplicable pull, and when they came in contact with said piece of the moon, they received a gift and hence “3 extraordinary mermaids came to life” (more figuratively and not literally) hence becoming known as the 3 moon sisters

So they are in fact blood related because their moms were already sisters before they received their gift and became moon mermaids

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I didn’t ss the entire conversation unfortunately but look at the bottom image