Is it possible for the questioner to be the product of an illicit relationship between her brother and her mother, based on her childhood observations of them? Does Islam recognize the medical proof of lineage? Is it permissible for the questioner to marry her sister's son if it is proven that he is not her brother by legitimate lineage?

Adultery is one of the gravest sins. With close relatives (mahrams), its ugliness is intensified. It is forbidden to accuse anyone of it without conclusive evidence. If you are certain of it, you must conceal it and advise your mother with wisdom and admonition. The delusions about you being the daughter of your uncle (the fornicator) are from Satan, for a child belongs to the marriage bed, and lineage is only disavowed through mula'anah (oath of condemnation). The woman is your mother, and the man is your father, and his children are your siblings, and the children of your sisters are your mahrams; they are not permitted to marry you. You must stop feeling lust towards any of your mahrams and avoid seclusion and physical contact.