Big sister April steps in to help her parents look after Elias.
Imagine tiny Subaru losing his first tooth and it just so happens to be his fang. Ayato teases him, calling him half the vampire and he panics over it so much so he cries on the spot. That's when Big Brother Shu steps in to show Subaru he lost a tooth as well (the alternate fang) and together they could be one super strong vampire. Subaru instantly calms down. They spend the rest of the day guessing how big the new fang will be when it grows in.