The Black Family Madness is just schizophrenia. Different forms in different folks. It shows up later in life and doesn't express itself the same way in all of them who do have it. But it's just schizophrenia.
When Sirius figures this out with his therapist in his early twenties, he freaks out. He doesn't want to end up like the rest of his family. His therapist has to hold his hand and get him to calm down before telling him there are things you can do to feel it less, to live normally. Medications to stave off the symptoms.
After that, the medicine cabinet in Sirius and Remus' flat nearly doubles in medication bottles. Alongside all of Moony's painkillers are now all of Sirius' "happy pills" as he likes to call them. In the mornings, they take their meds together. It's their little morning ritual.
When Sirius tells Regulus his discovery, Regulus gives him the most put-out look because he's known this since he was sixteen and has his own fun blend of medications he takes in the morning. They compare meds, and Sirius makes all the jokes he ever could about his brother needing one extra bottle than Sirius does.
