Hmm, I think I can see where this might lead. Tanuki is getting roped into being "lovers" with the new Red Flags girl and when he comes back home his sister will ask where he was while noticing that he was walking home with the girl. She'll get jealous and act like she was getting closer to his friend to try and make him jealous. The friend is going to get the wrong idea and take things too far and trouble will ensue since she's bad at handling pressure and tends to go along with things people ask her to.
I kind of hope I'm wrong because the friend could be a good character but they would ruin him if they just made him another horn dog asshole who only wants to bone the sister. He seemed to genuinely care about her from their earlier interactions and even in this chapter he mentions how she used to be different when he first met her so they have a bit of a rapport already. It could go another direction with her opening up more to someone besides her brother, but again the author hasn't earned that level of faith given their history.
Expectation is that the friend and the sister alone together at his house is going to cause a misunderstanding. Good ending is the misunderstanding is benign, bad ending is the friend gets character assassinated by the author trying to push a narrative.