Oh, they weren't superfluous earlier. They played a vital role in the opening episode, as the unkind girls of the kind and unkind girls. The king offering so much for their rescue was part of the heroine's motive to set out to seek her fortune. And rescuing them meant the heroine and hero had a claim on the king.
But now, they're superfluous. If they had done anything to merit dancing to death in red-hot shoes, or being abandoned in the wildwood as wolf-food, or rolled down the hill in a barrel stuck full of nails, that would be one thing, but they haven't. The rescue means they won't be marrying any cobblers for rescuing them. The hero has no living brothers, so the classic absurdity of their marrying them despite their behavior can not arise.
And as their great-aunt tartly observes, they already tried setting out to seek their fortune. They don't seem to be good at it.
So off brainstorming ways to dispose of characters. Running away because their behavior's been terrible is also out; it wasn't bad enough to fear punishment, and they wouldn't do it for less. Being abducted by an ogre, or dragon, or even bandits would in fact put them back on center stage, and they've already been rescued. Sticking them in a tower is only asking for trouble.
Traditionally, they can also be the villains for the last episode, after their sister married and has a baby, but I don't think that's consistent. One, at least, would certainly disdain it, because it's sneaky. And the other imitates her shamelessly
hmm. . . . perhaps they can be split up and have different endings.