(delurk)
Sophitia, being pure (a virgin), was charged by Hapheastus, god of the forge, to seek out and destroy Soul Edge, a corrupt and evil sword. Other gods (Aeries) also had a hand in its fate.
In her storyline, if you win with her, she settles down and gets married, and no longer feels the divinity of the gods. 9 months later, she has twins.
When the shards of Soul Edge still manage to cause problems, her younger sister Cassandra steps up to the role to save the family (hence the new character that looks and fights like her, but isn't her).
In SC4 in particular, Sophita's kids are held ransom, and she's forced to fight for the bad guys to protect them.
As far as BE goes... there's BE, and then there's changing character design for the sole purpose of sales / fan service. As much as I like BE (why else would I be here?), I have to agree that SC's ... evolution is disappointing. It's not like there's a REASON all the women increased bust sizes between each game, other than to drive sales. If they had worked in some storyline reason, I'd be more inclined to let it slide, but then I don't think it would have gone over well with the ESRB...
(relurk)