I don't see how these relate. Signal isn't explicitly aimed at high security targets, although it works for them (seeing the Snowden endorsement). Signal is about bringing encryption to the masses. Making E2EE the default choice for everyone. Is is much more about mass surveillance. Your sister has a different threat model, one where they are also concerned about the physical device being compromised (i.e. stolen, hacked, or being physically accessed).

These are different threat models. For E2EE for the masses you need things like stickers and for it to be "fun" in addition to being a tool. In your sister's threat model she's more concerned about the tool over the fun part. It is a work phone after all (I mean this is why they take out the camera).

E2EE for the masses is pointless if you can't appeal to the masses. Cypher nerds will always have their fun toys to communicate with but we are also in dire need for something that prevents mass surveillance. That is, after all, one of the fundamental necessities of a democracy: being able to speak your mind without fear of government spying/involvement (this is the reason they got funding from Radio Free America in the past). Unfortunately this means some compromises need to be made. But as far as I'm concerned Signal has done far better than any service I've seen and the relative leak is near zero. The weak points are SGX and pins, which only hide some minor metadata (even fully leaked this would be better than WA or Telegram).