What would be the impact on American politics if Joe Kennedy Jr. survives WWII and enters politics? Was he presidential material?
He was the one who was supposed to enter politics and win the White House for the Kennedy family, not Jack. Basically a rerun of JFK's OTL career: House, Senate, White House. He's somewhat more conservative than JFK and RFK, and was just as anti-Semitic as his father, in addition to being as compulsive a womanizer as his brother Jack. (When on overseas duty he had an affair with a married woman) Before the question is asked: RFK, not JFK, will be the "XO" as per OTL.
They can go into "civilian" careers. Jack was supposed to return to Harvard Law after the war if Joe lived, and Ted being "Fast Eddie" (read: parody of frat boy) at Charlottesville will probably occur as per OTL. Maybe RFK does better as an undergrad (not so much ability as willfully not putting in the effort IOTL) and goes to HLS instead of Charlottesville. It is worth noting that Robert was the only brother who never, and I do mean never, seriously contemplated a "civilian" career for his entire adult life.
Xen: As I said, much more economically conservative than a New Dealer, more along OTL JFK lines, which is equal to or even slightly right of Rockefeller.
W49: Ted is the best legislator of the 3 without a doubt. Other side of the coin is that Bobby is infinitely more ambitious (whenever Ted was asked to beef up a policy speech, he said "I'm not Bobby" IOTL), better suited to the Executive Branch and ideologically much more electable. That is where the "ruthless" thing comes in: like his fellow Dynasticrat Hillary, that sort of burning ambition and "my way or the highway" mentality scares the Democratic establishment to no end. SCOTUS I've heard before, and that would be interesting. He'd be a swing voting Justice Kennedy: socioculturally conservative on everything outside civil rights, an economic moderate, and pro-civil rights (1960s definition of civil rights).
So what do Jack and Teddy do, since they're basically out of the political running?
I happen to know that my fellow Kexpert Mav is redoing Kings of Camelot sometime in the near future, so all will be clarified then. There are no biographies of Joe Jr. per se, so you have to glean all the info from biographies of other family members.
*Among other things, Rose's memoirs are sanitized on this (among others) issue given her encouragement of sibling rivalry IOTL. Or as Jack put it: "I know Joe's their favourite."