Brian Laundrie's Sister Says She Doesn't Know if Her Parents Are Involved in Disappearance

Brian Laundrie's sister says she doesn't have the slightest clue where he is ... and she says she simply doesn't know if her parents are in on his mysterious disappearance.

Brian's sister, Cassie, confronted a group of protestors outside her Florida home Monday ... telling them she hasn't seen Brian since he went missing and she's been cooperative with law enforcement.

Cassie says she's just as frustrated as everyone else with the case, and wants answers.

As you know, Brian fell off the grid last month ... and his parents are the last ones who saw him before he disappeared.

Dog the Bounty Hunter is searching high and low for Brian, even venturing to a remote island off the Florida coast, but so far he's struck out.

There's been a few alleged Brian sightings ... most recently a hiker claims to have seen Brian on the Appalachian Trail ... but nothing concrete.