I'm currently procrastinating on studying so I went and tried to find out who in their family first went vegan and I found this very old video where Holly talks about it:
So first, Holly went vegetarian (she already had her ED at that point) and Emily "then jumped on the bandwagon" (to quote Holly) and went vegetarian as well. Then in 2016, Emily and Holly started to do more research into food and talked about wanting to go vegan. Then Emily watched a documentary called "Earthlings" and went vegan after watching that.
Holly wanted to go vegan too, but her parents did not allow it because they felt that Holly's diet was already to restrictive ("they just said no"). Holly tried to eat vegan as much as possible but her mom monitored what she ate very closely and she would always catch Holly (e.g. when Holly made her porridge with water instead of milk, Holly's mom would notice that the milk hadn't gone down so she caught her).
At that point, Emily had finished uni, came home, and started to talk to Holly's parents about the many benefits of veganism because Emily believed that going vegan could help Holly recover from her ED. Emily managed to convince her parents and they finally allowed Holly to go vegan. So then Emily and Holly went "full-out vegan" in "quite an extreme way", according to Holly. They followed a 'raw till 4' (?) diet. This changed Holly's whole relationship with food and improved her physical and mental health immensely (according to 2017 Holly, that is). Holly also said that she went vegan for health reasons but also enjoys watching documentaries about animals' rights etc because "it helps me justify my veganism".
Curiously though, there's no mention of their mom going vegan in that video?
But man a psychologist would have a field day with this video, I bet. But I lack the expertise and I really need to study now lmao