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Jegulus as messy exes will forever be my favourite thing so here enjoy some headcanons
James never told Sirius he and Reggie were dating (This was before the black bros started talking to each other again) so when Sirius and Regulus fixed their relationship, Sirius was amendment on his baby brother and best mate getting along but nothing seemed to work.
Regulus still keeps all of the things James gave him from when they were together. Often sleeping in on of James' sweaters because he likes the smell (but he'll never admit that of course)
James has the tendency to accidentally call people Regulus when he's snogging or shagging them
Modern/Muggle Au: Both James and Regulus randomly drunk call/message each other all the time which some how always ends up with them shagging and waking up bitter about it the next morning
Poor Barty and Lily are their respective rebounds, but neither relationship lasts for the same reason. James expects Lily to be like Reg and Reg expects Barty to be like James. Both relationships end on neutral terms (They stay friends) and in the end Barty dates Evan, and Lily dates Mary
Both of them speak horribly about the other but get extremely defensive if anyone tries the same.
Both of them are extremely jealous of any other romantic partners the other has. In turn both of the will snog random people in front of the other to make them jealous