My Stepmom is a Futanari 4

Your stepmom, Fiona, has a condition called hyperlactation.
It means that she lactates excessively and spontaneously.

It can trigger at any moment, and when it does, Fiona is desperate for relief.
Luckily she's been able to monetize her predisposition and turn it into a decent income to support the household and pay your tuition.

And though accidents happen, it's no use crying over spilled milk.