there are several ways to interpret this story, but first you MUST look at it from the hebrew point of view because of mistranslations.
there is physical interpretation,
first of all you need to make a distiction between hebrew bible and translated bible.
in hebrew bible (original) they lived, before apple, in "gan eden" or "garden of eden".
it is a fruitful, beautiful place. some believe for it to actually be around mesopotamia.
when they disobeyed god and ate the fruit of the "etz hada'at" or "the tree of knowledge", they were banished from this beautiful place and lost their holiness.
there is also the spiritual interpretation,
where "gan eden" or "garden of eden" was not actually a place but a state they were in.
both were the first humans and extremely holy, they are said to have immortality and was naturally as close to god as possible, but once they ate from the forbidden fruit, they lost everything and became normal.
some refer this entire story to humanity itself.
there are also others but thats the most general i can think of.
----------------------------------------------
what would happen if they did not sin ? that is depends on the interpretations you are going for.
whatever it is, they would still be [probably] immortal and holy.
they probably wouldn't have children and humanity would have stayed just the two of them forever.