Ellie/Julia or Sophia/Alyssa is
before the yoga event and
after you either take Dylan to boarding school or to his martial arts lesson. My guess is that there will be two different triggers; if you take Dylan to boarding school, Alyssa will invite her out there and then for a coffee or something like that, but if you take Dylan to the martial arts lesson she'll probably phone her to ask her. It will then likely be a choice between saying 'yes', then going to home to get changed, or declining and just going home.
In both cases, I reckon Sophia will see Ellie and Julia by the pool, but if you go with Alyssa then you won't see what else happens, and if you don't go with Alyssa, you'll see something steamy between Ellie and Julia which will trigger more lesbian incest temptation.