“Regulus would pick Sirius in a room full of people but Sirius would always pick James”
in a house full of monsters Sirius picked Regulus for years. James probably begged him to leave way before. It took Sirius to almost die to leave Regulus. Even then we don’t know if Sirius left or if Regulus sent him.
That one act where Regulus chose Sirius. Knowing what would happen to him just like Sirius did for years.
Do you really think if Sirius were conscious enough he would let Regulus do that?! Do you?
Sirius always chose regulus until Regulus made him stop. By his own will or his parents pressure we don’t know.
But no matter how many times Sirius calls James his brother, his mind will only think of one person.