The general consensus is that Artemis was born first and subsequently assisted with the birth of Apollo.
ðə dʒˈɛnɚɹəl kənsˈɛnsəs ɪz ðæt ˈɑːɹɾɪmˌɪs wʌz bˈɔːɹn fˈɜːst ænd sˈʌbsᵻkwəntli ɐsˈɪstᵻd wɪððə bˈɜːθ ʌv ɐpˈɑːloʊ.