Suspicious

"What's the matter?" Tim asked Rebekah, he looked a little surprised after hearing his sister cursing like that.

"Don't look back! Andrew is coming our way!" Rebekah responded panickily, the direction of the girl's gaze that was focused on behind Tim and how she was staring in that direction made Tim curious about what the girl actually saw and he intended to turned back, but he didn't do it after Rebekah forbade him and explained the reason for not to do it.

Tim himself is standing with his back to Billy's potato farm, while Rebekah is standing facing him, which is why the girl can see Andrew getting closer but not with her brother.