Ok so I was re-reading act 5 and there was a conversation between Aradia and Nepeta which revealed Nepeta didn't know Aradia was dead. So my question is who knew she was dead before the game and why did she want to keep it secret?

Aradia, Vriska and Terezi knew.

Whether Sollux or Equius knew is debatable.

And she didn’t tell anyone basically because no one asked. Ghost Aradia was really only focused on getting Sgrub going and didn’t really talk about much else.