(look at description for AU)

The new Blacks, you two were called. You and Sicinius practically were siblings, though being cousins. You were Regulus's child and he was Sirius's child. Thing was, he looked more like Regulus and you looked more like Sirius (head cannon is that Sirius is androgynous), though, you two and your parents were very different. For one, you were a Ravenclaw and Sicinius was a Hufflepuff. You two also got along very well, unlike the old Black brothers. You two were practically inseparable. Currently, it was the weekend. You, dressed in your weekend clothing, were chasing Sicinius, who had stolen your leather jacket from around your waist. You were catching up to him quickly, and just when you caught up and rounded the corner with him, smack. Right into a professor's chest. Not reacting much, you snatched the leather jacket back from Sicinius, who was now staring at the professors you both ran into. It was the newest one, Professor Lupin. You two hadn't had a class with him yet, but, rumours had it, he was the best teacher Hogwarts has had in quite a bit. Lupin looks down at the two of you, his eyes widening a fraction. You could've sworn you heard a jumbled mumble of 'Regulus? Sirius?' before he spoke up fully

“Oh! You two must be fourths years! No wonder I haven't seen you two around. Could I get your names? Just for class, of course.”

Sicinius's cheeks flush pink immediately as he opens his mouth to speak, but no words form. Looks like someone's got a teacher's crush, and it might just happen to be your cousin. Merlin, what is he, his father? You can't help but grin a bit at the thought of your cousin liking his basically non-offical step-dad, before resuming your hostile demeanor