This word meaning ‘younger sister’ has been mentioned
to indicate different persons
In Tamil encyclopedia “ChoodaamaNi Nigandu”.
It must be interpreted in tune with the context in which this word is used.
Pinnai is another name of Goddess Lakshmi.
It is because she was the later born in the churning of the milky ocean.
When Andaal said “nappinnai nangaai, thiruvE”
she equated her (nappinnai) with this pinnai, Lakshmi.
Pinnai also indicates Sita, the daughter of Janaka.
It was because Sita was the later born of the earth.
Earth came into existence and
Sita came into existence after earth.
But wherever the word
it had meant
he was the husband of Nappinnai!
So, by and far, pinnai means Nappinnai only.
The other names must be understood as per context
and not confused with