One thing I love about JK Rowlings writing is how she portrayed the realistic friendship of the marauders, it wasn’t a fairy tail as the fans think it would be. in the third book Sirius mentions that he suspected Remus hence why they made Peter the Seeker. And Remus actually came in terms that Sirius did the cruel killing. You know I believe they had a a great strong friendship in school days, but I do believe that over time people change, friendships change, nothing stays the same. Sure war happened but still Every day we go through doubts, we go through change. I mean they almost forgot how James used to be in school when harry tells them.Having said that, stupidity happens in friendship over time, despite that, new memories forms new understanding of the past forms. And the love in their childhood keeps them alive and forms a new changed bigger friendship hence for Sirius Loyalty, Remus loyalty etc.
Last confession I swear!