YOU SAID:

i was there before, since she arrived

INTO JAPANESE

彼女が到着して以来、私は前にそこにいました

BACK INTO ENGLISH

I've been there before since she arrived

INTO JAPANESE

彼女が到着してからずっとそこにいた

BACK INTO ENGLISH

she's been there since she arrived

INTO JAPANESE

彼女は到着してからそこにいる

BACK INTO ENGLISH

she's been there since she arrived