I tried looking for other things to support my take on why Leslie was tutored, but I didn't find much I could use aside from Chapter 2:
Marquis Sperado to Eli: "That child was born for your benefit. If you hadn't awakened your powers, I would've put you as a lady in waiting at the Imperial Palace." ... "If that child wasn't a sacrifice, I would've made her your maid."
Eli is 15 (from chapter 14), while Leslie is 12. Leslie was born long before Eli would have been able to awaken her powers (at least that's what I hope, imagine if Leslie had awoken these powers at 2 or 3 years old). If Eli didn't awaken to her powers, instead of being thrown into a fire, they would have kept raising Leslie to support Eli as a maid. I assume she's acted in that role in the past and having her knowledgeable is preferable (I'm thinking of the flashback where Eli punished her for mistranslating God's words).
Could also be other reasons behind it, like the research of past generations of the Sperado family including evaluations on how much power was transferred and how the sacrifice was raised (like how there were optimal times for doing the sacrifice).