Jimmy would have been cancelled for her today

The FACTS:
His mom delivered her and he saw her as a literal newborn as they were family friends
She became his little sister best friend and by everybody's words she was ALWAYS in their house growing up.
She mentioned that she didnt take him serious until after seeing him in a Naval Uniform when he started school there(She was 16).
They officially "dated" the one time he was able to visit home from the academy the summer before the end of WWII.(She was 17)
They married the next year as soon as he graduated from the academy(She was 18)

Now these are all the facts. A 20+ year old guy coming home to date his little sister's school friend would have got him cancelled by the mob today. Now, we can look at the assumptions.

The ASSUMPTIONS:
The Americus daily recorder newspaper reported, in an attempt to shame, the names of kids who were caught skipping school in the area. The full names of the boys were listed, while only the initials of girls were. Jimmy Carter is listed as having skipped with a few friends in 1940. The boys names are listed but there is an ES also there. Now there could have been any number of girls around town with that initial, but that was also the initials of Rosalynn before she married Jimmy(Eleanor Smith). Why would a guy who was a top student, very serious(as everyone noted), and never looking for trouble at all skip school to joy ride with some buddies and other girls like "ES"? Obviously because they had something he wanted there, and my bet would be ES. Apparently there was also an RC there, which matches Rosalynn's best friend(and Jimmy Carter's little sister) initials, but even then. Was Jimmy a guy who would let his little sister skip school just to tag along with his friends to get closer to ES? maybe?

Roslaynn states she didnt take HIM serious until he joined the academy and she saw him pictured in uniform. Then they were "dating" next time she saw him. The implication is that it took her seeing him serious for the ball to really get rolling and that he already saw her as serious. Jimmy has even mentioned himself that even on the date when he came back to plains he already knew he was going to marry her. Even by those days surely that wasnt a "first" date. Roslynn says she never let boys kiss on the first date, but he did. This implies she was dating while he was gone, but Carter himself said he didnt.

What I think the truth was:

Jimmy more than likely saw her as his girl sometime in the late 30's. At the very latest they were "dating" in 1940 when his friends decided to skip school to joyride across town. "Straight and narrow" Jimmy probably went along with it because his girlfriend thinks its fun. This is also the time her dad died from some form of cancer so maybe he was getting her mind off of things(although a 16 year old taking his 13 year old girlfriend out to skip school would be grooming today).

He starts studying engineering in 1941. I see 0 reason to think they werent dating during this time as he was going to the local college(Georgia Southwestern). At some point she is also taking classes here.

1942 he leaves the area all together to go to Georgia Tech. At this point I think she puts on the charm offensive. The period where she dates other guys that she does allow to "kiss" her after multiple dates and that is enough to probably piss him off and keep him coming home to check up on things.

When he's appointed to the naval academy from tech he can no longer come home so the stories about dating other guys to get him to always come back no longer works. At that point she views him as "serious"(16) and decides she's ready to settle down with him at the ripe old age of 17.

The rest is, as you say, history.