Pretty sure the point was for him to be there when the teacher tries something so he can rush in and "save" his sister, because otherwise he'd be feeding her to the wolves and the only after the wolf devours her would he shoot it which defeats the purpose of doing this whole thing to begin with.
I get the feeling it won't end that easily though, they set up that other girl like a chekov's gun and we're (apparently) approaching the climax of this pervert teacher arc already without her doing much of anything. She's going to play a role in this somehow and the obvious thing would be for her to have seen him infiltrating the school and following him. Where it goes from there is up in the air.
If she's actually an ally and genuinely likes him then she could be the one who interrupts pervert teacher and catches him in the act saving them the headache of trying to explain why her brother was there. If she's an antagonist then she'll distract or otherwise occupy Tanuki to the point that he misses the teacher molesting her or she'll steal his phone preventing him from gathering evidence. Regardless, she's going to appear and do something otherwise there was no point in introducing her for this arc specifically.