The thought probably never crossed Clint's mind that his sister was about to be abducted from outside their rural home. Never. So, when a couple of unusual things occur -- the dog barking and voices in the garage/carport -- it alarms him a bit. But still, it probably doesn't occur to him that his sister could be in trouble. He might even think -- at this point -- that his sister has left for school.
The fact that he is not alone -- as he might have expected -- bothers him. Does the thought cross his mind to go outside? Maybe. But he might've thought he'd be outnumbered 2-to-1, if he did. Besides, it could be nothing. So, he calls his mother to find out if, by chance, she had expected someone at the house. Again, the last thing he expects to see is the violent abduction of his sister from her home.
So, his mind tries to piece together the few details: two figures bending over, camo attire. It must be Drew. He must have shot a turkey. Then he sees the two of them -- one of them being his sister -- heading toward the woods. Holly shows no signs of being in distress. So, Clint probably assumes that he, in fact, was right: It was Drew who had stopped by. Again, he has no reason to think that his sister is about to be violently abducted.
I can better understand Clint's behavior, thanks to this interview. Clint tries to respond to an unsual situation. He perhaps is fearful enough not to confront the stranger(s). (He might not even be dressed.) But he doesn't want to overreact either. So, he calls his mom, to see if she knows something he doesn't know.