>>42282781
Rainbow Dash is not an only child. Derpy is her sister, but this isn't acknowledged by their parents.

This is a closely held secret within RD's family; her parents are so invested in their superstar daughter that they're embarrassed to admit that the disabled(?) Derpy is theirs. RD goes along with the charade for her parents' sake, but continues to look out for her goofy sis.