If Lyra chose to retreat, she could forever monopolize her sister and become Astrid's only lover.
This ending would clearly satisfy Amalia's twisted possessiveness and bring her temporary happiness. The downside, however, was that Astrid would undoubtedly fall into prolonged melancholy, possibly never recovering for the rest of her life.
When you love someone, your own emotions inevitably shift with theirs.
Amalia wanted to be her sister's sole partner, yet she didn't wish to see Astrid become despondent because of it.
Just imagining Astrid sitting at a desk in the future, nostalgically reminds the once deeply intimate pink-haired maid and silently grieving, would bring an overwhelming bitterness and guilt crushing down upon Amalia's heart.
As the primary cause of all this, she bore the majority of the responsibility.
