Full twist: she is a girl with a very troubled life story including witnessing the brutal death of a dearest friend and all, and in private she suffers from debilitating clinical depression and PTSD from that. All her tsundere tics are just some elaborate cover-up to hide and distract others from noticing her immense insecurity and mental instability. A fake cat ear just further drive this home.
In short: Pet her more, talk to her more and make her happy.
Full twist: she is a girl with a very troubled life story including witnessing the brutal death of a dearest friend and all, and in private she suffers from debilitating clinical depression and PTSD from that. All her tsundere tics are just some elaborate cover-up to hide and distract others from noticing her immense insecurity and mental instability. A fake cat ear just further drive this home.
In short: Pet her more, talk to her more and make her happy.
And Sims, despite being a troll/prankster, is a good elder sister and follow suit so she doesn't feel left out?