I have to say, that I am glad that you are taking in a rehomed bird as oppose to buying a bird at a big chain store. Big stores treat their animals horribly, and you don't want to promote that kind of business.
I'm glad to see that you are offering a bird in need a home, but even so, I can't help but worry... I understand that you're probably still mourning the loss of Kiwi and feel that having a new bird in your life will help with that loss... I know that what I'm about to say will probably be immensely unpopular, but can you responsibly take in another bird in your life right now, given Kiwi's fate? It sounds like even when you know what is right for your bird and you know what the right decision to make should be, you are surrounded by people who hinder your efforts, and Kiwi suffered for that.
So, I have to ask: what will you be doing differently this time so that this new bird has a long, happy life with you? What will you do this time to ensure that you can protect this bird? Will you lock the bird cage's door so that your sister does not put this bird's life at risk from your other pets? Do you have the money saved up so that you could pay to Uber/taxi to a certified avian vet and pay for that vet visit in case of an emergency? Will your new bird go in for an initial vet check up (and also so that your vet already knows you and your bird before any emergency situation happens)? Will you be able to continue taking care of this bird for the next 20-30 years of your life?
I know this may be hard to hear, but I never want you to relive what happened to Kiwi. You may hate me for saying all of this, but I hope that you see it for what it is - a little tough love.