She's going to shove that huge lollipop up your ass
Might as well make Sage half-manatee at this point
Small world may be a rule but it's not the only one. Sage's biological mother being someone we've already met would certainly be within the rule, but we are low on candidates.
I entertained the thought that Sage and Zoey were half-sisters, but Sage was five when she was put in foster care so Zoey was definitely born by then and would have been put in foster care too.
I am confident in ruling out the manatee on the implication that Sage's mother gave up because of her age and the manatee couldn't possibly have been too young or too old at the time. Too young makes more sense, but I don't think too old can be completely ruled out. In the latter case she would be one of the senior ladies. Someone who did her best for five years and then a diagnosis for something sufficiently dramatic forced her to give Sage up. The former would imply high school age. A girl who tried her best for five years and then ran out of resources. Someone who now would be close to Jade's age.
I'd rule out every mom on the same basis I rule out Zoey's. She would have given up both or kept both. If Sage's mother had children after giving her up they'd be high school age at most. Bella and Cathy would be the right age, but for so many reasons they just aren't possible. Everyone else is way too young.
I'd be more inclined to believe that Sage's mother is about to turn up. Quite possibly as the new teacher.