Answer:
The narrator thinks that the wallpaper is having the same effect on them as it is on the narrator.
Step-by-step explanation:
The strange behavior of John sometimes and Jennie's inexplicable looks made the narrator think that this is because of the wallpaper and the narrator even caught John and Jennie both several times staring at the paper and the narrator also caught Jennie with her hands on the paper. Hence these events lead the narrator to think that they are having the same effect from the wallpaper as the narrator.