obviously sirius’ is his leather jacket. i really like the hc that it was remus who bought him for his birthday, but that’s not the only reason. it’s the very solid proof of his punkrockness and rebellion. he would have tons of pins, from bands and against discrimination ( not only blood predjudices but also sexism, homophobia, racism, etc… ). once james spilled his pumpkin juice on it and sirius fucking cried.
remus’s is not ( surprise ! ) a jumper. it’s his old gryffindor scarf. of course the guys call him a nerd when he says that, but it’s true. it’s the symbol of his humanity. it’s a normal item, that everyone has, and therefore makes him feel normal. it’s also the reminder that despite the discriminative laws, he was able to go to hogwarts and find friends.
peter’s is his acdc’s tshirt that he got at a concert he went with sirius. neither james nor remus could go so they had a good time together, and peter didn’t feel like he was less of a marauder than the others.
and james’s is, of fucking course, his quidditch uniform. he secretely wears it on weekend, when no one is watching. in seventh year, he learns that lily secretely loves it too and he starts wearing it just for her.