Who assisted with her brother Apollo's birth when she was just born herself?

Diana, also known as Artemis, is the correct answer because she is known in mythology as the twin sister of Apollo. According to mythological accounts, when Leto, their mother, was in labor with Apollo on the island of Delos, it was her daughter Artemis, who was born first, who assisted in the birth of her brother. This showcases the close bond between the twin siblings and highlights Artemis's role in childbirth and her protective nature, as she later becomes associated with women's health and childbirth.

The other figures listed have different connections in mythology. Athena, the goddess of wisdom and warfare, is not involved in the birth of Apollo. Hera, the queen of the gods, has her own tumultuous relationships with other deities, particularly with respect to her animosity toward Leto, and is not part of Apollo's birth narrative. Demeter, the goddess of agriculture, also does not play a role in the birth of Apollo and is primarily concerned with fertility and harvests. Artemis (Diana) stands out as the one who directly assisted in the significant event of her brother's birth.