Ellie needs an external element to move forward in her relationship with Sophia.
the kiss of Julia and the development of her lesbian side, knowing that Christine and her mom were showering together, having seen Sophia & Patricia kiss, knowing that Sophia found her dildo, having seen the scene between Sophia and Julia, ...
I think if she sees or learns something again, it’ll push her to take a step towards Sophia.
For example : if Ellie surprises Sophia and Nathalie, she could ask Sophia to do the same. Nathalie may have a passive or active* role.
*If Nathalie sees Ellie watching, she could make Sophia say naughty things to excite more Ellie.