@Solaz : One idea that made me chuckle imagining it is like this:
Dad goes off deep end, try to put Eli in a fire trying to get her to awaken as much power as Leslie (this is after Leslie is adopted by Salvatores)
Eli either escaped or was about to get BBQ'd when the Salvatores came to save her, Bethrion leading the charge.
This resulted in Eli forming a tsundere crush on Bethrion, and use 'visiting Leslie' as excuse to go to Salvatores house to see him.
But Bethrion is oblivious to this romance thing and thus each time Eli try to switch conversation to Bethrion, he always talk about 'muh imouto da best' (Leslie), much to Eli's chagrin.
And Leslie who noticed Eli's feelings could only smile at these two's awkward path towards romance and try to welcome Eli whenever she's here for a visit to help her out (Eli being a tsundere might try to use 'I helped you with the carriage incident' to try and get Leslie to help her)
In a way, Leslie forming a cordial relation with her sister might give rest to the souls of the sacrificed children.