Curate, connect, and discover
When regulus told Sirius he was trans, Sirius practice told James immediately. Sirius only told James so he could protect regulus. James wasn’t a violent person but when it came to someone he loved he would do anything to protect them. At the time Sirius didn’t know James loves regulus, at the time Sirius didn’t know regulus had already told James, had already asked him to protect him, indirectly of course. Sirius only figured it out when James punched someone for insulting regulus, Sirius only figured it out because James would do anything for someone he loved.