Look Sirius may get the highest grades but it’s a common known fact that he was not the brightest when it came to common sense but I’ll give you this: he was very good at reading people. Which is why after a couple weeks of noticing Regulus’s behaviour he figures it out.
It wasn’t just the stolen stares across the great Hall or the fact that they were always bumping into him now, or the way that he would sneer a little less when Sirius was with James but it was the fact that it he looked generally happier.
Regulus liked James.
So one day Sirius grabbed his arm tightly and pulled Reg aside, catching him off guard, his grip tightened as he spoke, eyes freezing over.
“just know that if you hide it doesn’t go away”, obviously Reggie faked ignorance and pretended not to not know what he was talking about and walked off.
Sirius knows he should’ve comforted him, made himself out as someone Reggie could talk to but he couldn’t help but feel jealous and bitter. All his life he’d been told that him and Regulus were so different. That Reg was going to be some great Heir that was gonna do so much more than Sirius could have ever done. That Sirius was nothing compared to him, just the disappointment. And he knew that Regulus would never come forward to his parents about the way that he felt. They would live the rest of their lives not knowing that Regulus was just like him.
