How can we get my autistic sister to understand about gratitude and doing things in return?

Let’s face it, we’ve all done SO MANY nice things and given her things and all but not all the time. She should be grateful. She says thank you. She’s autistic so she doesn’t always see the point fast enough. She gets bitter and upset when we asked her what did she buy us because let’s face it, we have done so much for her. The least she can do is do something nice for us for once. She never did. She always goes to the store and buys a lot a snack. A lot of snacks that is the size for a family. Dad was like, “What did you buy us?” He was kidding. I know she isn’t obligated to buy us anything but it would be nice if she did something for us. Our cousin got her a small fish aquarium as a surprise. She thought I was for everyone. He did a lot of nice things for her but she never did anything nice for him. I told her, “You know he’s done a lot for you so you should do something nice for him.” She looked down and said, “Yeah…sure.” He told her not to forget him when she went out to the dollar store. She ended up buying him something for once. Finally! Our aunt said, “You shouldn’t bite the hands that feed you. You need to start thinking about others instead of yourself.”

I won’t lie, she’s a spoiled brat. It’s true. She gets bitter when someone says she should buy others something or do something for them. I can some relatives said she’s too spoiled and always gets what she wants. She doesn’t even pay bills at the Josie. Dad should really start making her pitch in and help. She’s 19! She doesn’t even help around the house. She does nothing nice for dad either. Dads girlfriend did a lot of things and helped her a lot. My sister told me she’s taking her out for a day of shopping. She thought it was all about HER. I told my sister she should buy her something since she’s always doing nice things for her. She looked at me like I was crazy
How can we help her be less selfish?

How can we get my autistic sister to understand about gratitude and doing things in return?
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