this is a random thought bc i got angsty, so!
walburga, who āin a mad attempt to stop regulus from becoming like sirius after he runs away from homeā, obliviates regulus.
not completely, of course, but he makes him believe that he never had a brother in the first place. regulus is now the sole black heir and that's everything he knows; i don't put it past her to erase most of the abuse she caused, but still left enough to make him scared to disobey her.
and, like, it takes everyone too long to realize it.
first, sirius is not surprised his brother doesn't look at him anymore and pretty much pretends he doesn't exist, their bond has been severed completely to him, even if it hurts. and, well, he sort of wanted to get rid of his last name completely, so he pretty much asked everyone to not call him 'sirius black' ever again. so, how could regulus suspect anything.
then, everyone in slytherin knows not to talk about sirius in front of regulus (because he used to jinx them almost to death for that), and, sirius was officially disowned by the black family, so they do speak about regulus as the sole heir, as if sirius wasn't ever in the picture.
it's maybe pandora who finds out first.
barty and evan aren't sirius' biggest fans, and they don't like to confront regulus about how he feels, unless he's the one who wants to vent. they think that his way of coping is pretending like nothing happened, and while that's not healthy at all, regulus looks fine, super fine, even. so why would they want to interrupt his peace.
dorcas has been distancing a bit from them, because of the whole voldemort situation. she's no longer with them enough to realize regulus has been acting strange.
and pandora was a bit like evan and barty, at first. especially because, she knows regulus doesn't keep anything that's really hurting him for long. not to them. they just like to give him enough space to process and cool down. they let him come to them, and they will be waiting.
but then, regulus doesn't.
so, after a while she decides to ask him about sirius.
when regulus just answers "who's sirius?" with the most genuine and confused face on earth. she knows exactly what has happened.
i haven't thought further into it, maybe evan, barty and dora argue about what to do. the right thing would be to help him get back his memories. but regulus looks so much better without the heartbreak sirius has been unintentionally causing him since first year. and sirius seems to love his new life as the potter's ward. maybe that's the right thing.
but also, they know that, even with all the pain that he has caused him, regulus loves his brother more than anything and wouldn't want to just be forced to forget about him.
yeah. that was my random thought of the night. toodles!