Is it permissible to act based on doubt concerning the lineage of a brother's child, and what should be done regarding the brother who is unaware of the matter?

Accusing a person of indecency without proof is one of the gravest matters and a major sin. Allah Almighty says: "Indeed, those who accuse chaste women, unaware [of the evil] and believing – are cursed in this world and the Hereafter; and for them is a tremendous punishment." Allah has set strict conditions for proving adultery out of concern for concealment and the preservation of society. A child is legally attributed to the bed (the husband), even if the mother's adultery is proven, and can only be disavowed through Li'an (oath of condemnation). Therefore, one must be cautious about delving into this matter or speaking with the brother about it.