Freud was an expert on sibling rivalry, it is a well know phenomena.
Doesn't make it any easier to cope with or to bear when the sibling dies.
Your sister chose you as her safe space for anger and whilst that has been uncomfortabe for you, it is also flattering, that you were the one she trusted with that part of her struggles.
She did seem jealous, that her daughter had you as a favourite, try to think that at least she told you that, she was acknowledging your worth as a person and what you meant to her own child. She was being honest with you, she could more easily have made up some spiteful nonsense about no one liking you.
She was in a dark place and seeing things through the gloom but still she gave you that compliment and truth. That was a big and brave thing for her to do then.
Her hurt from her own illness was projected on to you, because she valued you and was able to share that stuff with you, she could not do that in ways you recognised as her trust, but I think that was it.
Light candles for her, send up prayers, keep her memory positive for the sake of your family.
Love is a tricky bastard, we don't see it in all its forms but I am sure she loved you. I am sorry that she died alone, that is a sadness and I am sure you would have been there for her given the opportunity, be gentle on yourself now.