1. Sis is clearly not coping with guilt and loss well at ALL. But the solution isn't to spend more money on LW's daughter, it's probably to spend more time in therapy. LW needs to... I don't know, get her own therapy if she can't let this go.
2. Tantrum is such an ill-defined term. "Tendency to tantrums" can definitely include a lot of behavior that's age-typical for 12 year olds. They're a lot like toddlers, except they're better at arguing and you can't manhandle them anymore. That said, this all sounds like it's been hard on Stepsister, especially if she is out of step with her peers or otherwise has trouble making friends.
But this advice... I mean, there isn't much advice you can really give to LW, because "Your dad and stepmother need to find Stepsister some activities of her own that will hopefully expand her social circle without intruding on Sister's space" is not advice LW can meaningfully take. That doesn't mean that this advice is right or fair or actually does anything to address the problem here. It's perfectly reasonable for a young woman to prefer her sister over the stepsister she only met 2 years ago, and has never lived with. Anybody who'd expect LW to not do this is... I don't know.
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2. Tantrum is such an ill-defined term. "Tendency to tantrums" can definitely include a lot of behavior that's age-typical for 12 year olds. They're a lot like toddlers, except they're better at arguing and you can't manhandle them anymore. That said, this all sounds like it's been hard on Stepsister, especially if she is out of step with her peers or otherwise has trouble making friends.
But this advice... I mean, there isn't much advice you can really give to LW, because "Your dad and stepmother need to find Stepsister some activities of her own that will hopefully expand her social circle without intruding on Sister's space" is not advice LW can meaningfully take. That doesn't mean that this advice is right or fair or actually does anything to address the problem here. It's perfectly reasonable for a young woman to prefer her sister over the stepsister she only met 2 years ago, and has never lived with. Anybody who'd expect LW to not do this is... I don't know.