It is not illegal for a son-in-law not to support his parents in law. The law does not stipulate that a son-in-law must support his parents in law. The Civil Code merely stipulates that adult children have the obligation to support their parents when their parents lack the ability to work or live in difficulty. Therefore, it is not against the law for the son-in-law not to support his parents in law. However, a widowed son-in-law who has made major contributions to his parents in law may inherit the estate of his parents in law.
 [Legal Basis]
 Article 1067 of the Civil Code states that if parents fail to perform their obligations of upbringing, minor children or adult children who cannot live independently have the right to require their parents to pay maintenance fees.
 Adult children who fail to perform their obligations of support, parents who lack the ability to work or who are in difficulty in life, have the right to demand that adult children pay alimony.