Is it permissible to purchase a property left by the family of a deceased man whose heirs are: a wife, a daughter, an adopted son, and brothers and sisters, knowing that civil law does not allocate anything to the siblings? And is this sale considered a sale of usurped property?

It is not permissible to buy this property from the two sons and their mother, because the two sons are strangers to this wealth and have no right to dispose of it. The wife is a partner and does not have the right to dispose of the entire house except with the permission of the partners. However, if all the legal heirs—namely, the wife and the brothers—agree to sell this property to you and divide its price, then you may buy it from them, and there is no harm in that.