What is the ruling on a mother who denies her daughter in front of people and claims that she is her sister's adopted daughter, and that she was forced to raise her after her sister's death?

It appears that the woman who denies being your mother is your paternal aunt, and she raised you. There is no blame upon her for informing you that you are not her daughter; rather, this is the correct thing to do. It would be wrong to say that you are the daughter of her maternal uncle because this contradicts the truth, unless there is a significant benefit [in doing so], and it is preferable to use tawriya (ambiguity). If your paternal aunt breastfed you five times during the first two years [of your life], then she is your mother through rada’a (nursing), and her children are your siblings through rada’a, and it is unlawful for you to marry any of them.