My personal take on the James-Younger gang has been that Frank and Cole were the brains, and to an extent the more ruthless members. Both had been in the war longer than any of the other members, saw more, did more and probably suffered more effects. I think they allowed Jesse to be the voice/bluster and happily sat back in more anonymity.
I think Jesse is a classic case of what happens to child soldiers as he was ~15 when joining Bloody Bill's band (Africa is unfortunately full of them at the moment).
My support in this is I do not believe there is a record of Frank participating in any activities after Northfield. From that point forward, Jesse is in "control", but becomes paranoid, killed a gang member, and trusted the wrong people. Just my $0.02 on them.