In my fanfic, Apollo is the name of Diana's twin brother, who only makes appearances in the future (also not yet mentioned in the fanfic).

However, Phoebus, the name of Celeste's younger brother is an alternate name for Apollo (in real mythology). This is because Phoebus Apollo is the grandson of the moon goddess Phoebe.

Mitra, the name of Celeste's younger sister has special significance. According to a baby name book I looked at, in Hindi it is the name of a god of daylight. And in Persian it means "angel." (note: if anyone who knows more about these cultures than me says otherwise, let me know).

Celeste's name means "heavenly" in Latin

Angela, Celeste's older sister, also is a name significance. In Greek it means "angel" or "messenger."

Hyperion, Celeste's oldest brother, is named after their grandfather; in mythology, Hyperion was a titan of the sun.

Celeste and her four siblings each also have Stellarian names; in a later chart, the translated version of these names are available.