That is not accurate enough ..
You didn't state the process or machine you used thus far, other than a sander. Woodworking is often about precision cuts and accurate dimension, and half laps are no exception. Equal amounts of material need to be removed from the flat surface or face and the depth must be also be equal. That's not the case in the photo I see.
I normally make those half laps using my bandsaw and a fence with stops for repeatability. Using a handsaw like a Japanese pull saw or a stiff backed saw would be my chosen hand tool. Then a sharp chisel could be used to pare down in small increments for an exact fit. Sanding the material is NOT the correct method to make accurate joints.
What was your process?
Thin wood makes accuracy even more important. A router in a table mount would be an easy and accurate way to get both surfaces flat and of equal depth. I am assuming you do not have access to one...?
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 10-16-2018 at 10:31 AM.