Wow I surely feel your pain and am so sorry you had to go through this. But, I agree with those that said what goes around comes around. It often does.
Over the many years of my life I gave my mother a lot of things. And so did my dad. She gave every single one away. I asked my dad once why he kept buying her a watch for Christmas knowing she was going to give it away shortly thereafter. His answer was that it wouldn't matter what it was.
Another example: I crocheted an afghan for her in colors that matched her room where she liked to sit in a recliner and listen to the radio. She gave it to my brother and said she had it made especially for him. I tried to explain to him but he did not believe me.
Anything of value or family history was given to my brother or others. But, I still took care of her at the end of her life and can sleep easily at night knowing I did the right thing.
A sweet Christian friend, now deceased, once told me, "This too shall pass." I didn't quite understand that at first, but as time went on, it became a great comfort to me to think of that.
So, I pass those words on to you along with a hug for you and your sister.