I remember there was an expression like that used informally, but I can't quite remember what it was. I remember expression like "to scratch each other's back", but it's not quite that that I am looking for and it's something you can plug into almost any sentence and that always sounds good like "to one up one another". It's not "in solidarity", it sounds way too formal. I think it implied that they would get something back like the expression "to scratch each other's back", but it's ok if your idiom doesn't imply that.
Seeing he was drowning from fatigue, his brothers all flocked to him in order to ___.