I don't really remember any tidbits we got about the empress's story before, so if I'm saying something already confirmed/refuted I apologize in advance, but mostly based on his chapter, here is my two cents;
How does that famous quote go... "cool motive, still murder"?
So now we see why the empress & little bro "went bad", but honestly I find it a little hard to believe that someone kind and good, as her brother called her, gets all wrapped like this just because a man she didn't love (as it was a political marriage) had a child with someone else. If this was a love-match I could understand a bit better, but this is a little excessive for me. Her brother, too. I get being traumatized by these events, but he seemed like a nice kid, and now he's like...this. I really hope that if we're already going down this route that they flesh this backstory more.
For example, if you reveal the emperor's whole marriage with the empress was BECAUSE he wanted Ceasar's mom as his queen and him as his heir (like, he loved her or something). Since the text says "barely any time after my sister and the emperor were wed..." the maid was pregnant long before the marriage. Unless the two were engaged for a long period of time (which doesn't appear to be the case, but I may be wrong) It seems logical to assume this whole marriage was BECAUSE she was pregnant and no one would let a maid be empress, but if there was a proper noble empress on the throne there will be less opposition...
Now THAT, being used not only by your family but by your husband as well, all for the sake of other people's desires and emotions... that is a much more fleshed out reason for all of this.