Sometimes 姐 and 姊 are interchangeable, sometimes 姐 and 姊 have different meanings.
Can we summarize their usages? What is their etymology? And can we find their English equivalences?
It looks that 姐 has something to do 祖 with the same 且. The 且 has something to do with the tombstone or the male reproductive/sex organ.
But it seems counter-intuitive that:
姊 is used for the senior sister.
姐 can be used for both junior or senior female.
小姐 means Lady. Like "各位小姐先生" means "Ladies and Gentlemen."
(祖, in contrast, is always used for the senior person or the ancestors.)