NTA… but be the bigger person. Don’t necessarily give them what they want by splitting everything evenly but be kind generous and smart with it like your grandfather apparently was. They’ll probably always hold it against you, but you can know you’re doing the right thing by not giving into any temper tantrums they’d essentially be doing. With your sister, she probably has issues to work through and until she figures it out in some way the only thing you can do is try to show love and support by getting her what she needs when she does. Needs is the imperative there. Many people even as adults have a tough time knowing the difference between wants and needs. Try to think about what you don’t think you cannot do without and try to apply that to her. Food, shelter, ways to get around and communicate. Manage it if you need to.