Moon Children (Undertale/MLP Crossover

Luna had married Crossbones (sans) a few years after the monsters got released from the underground. They had fraternal twins, a boy and a girl. But, Crossbones and the eldest twin got separated by the Monster-Pony War. Cross had to flee with the eldest to keep the youngest safe.
Soon, the twins grew into full grown alicorns and Crossbones and Luna are together once again. It took awhile, but their son and daughter loved each other like siblings.

It was a happy life, until tragedy struck again.
Crossbones had to flee to the underground to fight off an 'Impending Danger'. He said he'd return. But he hasn't yet.
The twins lose their mother to the danger, their mother isn't dead, but lost in a void.
They'll have to work together, the twins, to save the underground and Equestria

But how?

The youngest is blind and hasn't learn any magic due to being under protection. Hell, she doesn't know she even has magic. The elder one thinks that the youngest knows, but only to find confusion in those pale blue eyes.