Here is the full list of all 30 Tell Me Why achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Lock-up and leave the Ronan House with Tyler
Complete the story with the twins strongly bound together
Catch all the fish with Michael
Learn the truth about Sam's relationship to the twins
Find out who the Mailbox Bandit really is
You completed Chapter Two
Make peace with Mary-Ann at the cemetery
Win the plush battle
Discover intel about Mary-Ann's past and her death
Find a recording of the twins' 911 call
Correct the old graffiti on the tree
Figure out who inspired each Book of Goblin character
You completed Chapter One
Remember the night the twins agreed to lie
Find the password to unlock the Police Station computer
Make Tessa tell the twins everything she knows
Solve the puzzle on Mary-Ann's door
Enter the Ronan House
Leave Fireweed Residential Center
Find your first hidden Book of Goblins figure
Take time to talk things over with your twin.
When we say locked, it's locked
Stick a note on Tyler's back
Prove to Tyler that moo boxes are actually funny
Find three classic posters
Start writing a new chapter with Alyson
Uncover the princess' lost treasure
Complete the story with the twin's bond weakened
Place all collectibles in the attic
Help Sam fix the engine