Ok, let's look at the facts
The battle between Sparda and Mundus, ending in Sparda being victorious and shutting the gate to the demonic realm, was 2000 years before. Sparda lived long enough to meet and marry Eva, who was human and alive in the more recent years, as there is no indication that he met her in the past. and there is no indication that Dante and Vergil are hundreds of years old.
Dante looks like he ages normally, but at a certain age he MIGHT slow down. Which is probably why he doesn't look much older in DMC2.
Sparda probably didn't lose ALL his powers. Most likely he retained some amount, after sealing away a majority of them. Arkham had strength and speed, even though he did not have Sparda's power. The same goes for Vergil and Dante.
Anyways, so my educated guess ( not a theory, lol) is this:
Sparda fought and defeated Mundus, sealed away the demonic realm as well as a majority of his own powers. He lived alone for almost 2000 years and fell in love with Eva in the not-so-distant past. He married her and had Dante and Vergil. As everyone knows, kids sometimes relate more to one parent that the other. Vergil probably had a better relationship with their father than Dante COULD, even though Dante wanted to. and Dante related with Eva, hence his closeness to her and Vergil's possible jealousy and path to achieve ultimate power. However, Mundus broke free, and Sparda vanished. although I do not know for sure, I'd guess that he had tried to prevent Mundus from breaking free by going back and trying to re-seal him...and he failed....and died in the process.
So there, that's my guess
