Sigil: City of Doors

The rakasha sorcerer prestige gains a familiar, I have a few questions. What does this familiar do? Does it provide a bonus? Overwrite your own familiar? Also do familiars fill the henchman slot or their own slot?

Well, you can only ever have 1 familiar. And, iirc, they get their own slot.

Familiars exist in their own slot. You can have a Familiar, an Animal Companion, a Henchman, and a Summon up all at the same time.