Can my sister legally take everything from my mother's assets without showing a will to me?

My mother passed a way a couple of wks ago from a UTI that may have resulted in malpractice. My sister and I, although are mostly estranged, are trying to seek justice for our mother, but in the mean while my sister has told me she owns everything that my mother has owned since she mostly took care of her since she was two house down as I lived in NYC for years. I can say that I'd beg to differ since whenever I visited my mother (almost always) my mother was dirty and the house was in a bad state while my sister's two kids (who lived with my mother) rarely picked up after themselves. This has been an going agony for me for years and I've argued with my sister over these issues. I've called agencies to try to help my mother and her house and social services over the kids not being homeschool at all. I feel that I'm dealing with a morbidly narcissistic sister who has always seemed to get her way with anyone in her family as well as my mother and I.