When Bri recognizes that it's entirely possible that Jay could be back on drugs, it shows that she still doesn't entirely trust Jay to stay sober and in her life as a functioning parent. Again, this gives the reader the sense that Bri is juggling all sorts of anxieties and trying to stay afloat amidst all of it. She doesn't feel entirely safe and secure in her world because of that, which in turn makes it harder for Bri to concentrate in school and makes her even more desperate to gain independence by pursuing rapping.