This is in big part guesswork as it's hard to know exactly happened, but here it is:
We know they did a Bet against heaven. In exchange for 'something', the Parents were retroactively retconned out of the timeline, and Su Luo has to find 12 artifacts which have the Father's vassal Kings spirit within. If she manage to, she will get a key to the next step where the Father is imprisoned.
This means that Su Luo cannot talk about her parents (nor can any anyone else) because the Heaven might hear and retcon her out too. There are some places that are shielded, but they are very rare.
As to the mother, it seems she managed to reincarnate herself in order to prepare the path for Su Luo. She is also probably the Su Sovereign.
Now, personal theory: We know that no one is perfect in the setting. The stronger you are naturally, and the bigger flaw you have somewhere else. However, Su Luo seems to be a child fro 2 Sovereign (which probably shouldn't be possible) AND she doesn't seem to have a big disadvantage like the Tortoise' Son.
So, my guess is that the Bet was in order for Su Luo to be born. Furthermore the splitting of soul/etc she got was also in order to maximize her potential.