Goosebumps: The Game

To get your brother to appear, you need to deal with the creature in the mirror in the attic.

To access the attic, head right at the top of the stairs, then into the room directly in front of you. The walls will start shrinking, so you need to act fast! Place the stepladder (found in the basement) on the floor. Then, click on the attic to open the hatch. Click on the attic again to lower the ladder. Then head into the attic.

(Note, you may find it impossible to get into the attic without being crushed by the walls. In two playthroughs I got through okay once, and died repeatedly on the second. If you can't enter the attic, you need to use the screwdriver on the pipe in the shrinking room, causing goo to engulf the walls. Then, use gargoyle saliva on the goo to turn it into stone, allowing you to pass safely. Gargoyle saliva can be found by locating the gargoyle outside the window of your parents' bedroom, and using the empty container found in the photo development room to collect it.)

Once you're in the attic, use the hand mirror found in the bathroom on the creature in the mirror, which will cause the mirror to shatter. Pick up the Goosebumps book left behind. When you back out of the room, you will be back downstairs, where your brother will appear. He will ask you to get his birth certificate from a chest in your parents' bedroom, and gives you the code to open the chest. Go and collect the birth certificate, then return downstairs and give it to him. Talk to him and he will go outside. Head outside, talk to him again and you can now choose to go to the mall, unlocking the achievement.