Regulus had been living with the Potters since he was eighteen. Even though James and Sirius had already moved out by that point, Mia and Monty, James’ parents, were overjoyed to welcome another Black into their home. Much to Sirius’ surprise, the Potters and Regulus formed an incredibly strong bond. When they became sick, Regulus was the one who took care of them. James and Lily moved in after Monty passed, so that Mia would be surrounded by family in her final months. The three had been living together ever since. 