Section 20. Any person, over eighteen, who, being possessed of sufficient means, unreasonably neglects or refuses to provide for the support and maintenance of his parent, whether father or mother, residing in the commonwealth, when such parent through misfortune and without fault of his own is destitute of means of sustenance and unable by reason of old age, infirmity or illness to support and maintain himself, shall be punished by a fine of not more than two hundred dollars or by imprisonment for not more than one year, or both. No such neglect or refusal shall be deemed unreasonable as to a child who shall not during his minority have been reasonably supported by such parent, if such parent was charged with the duty so to do, nor as to a child who, being one of two or more children, has made proper and reasonable contribution toward the support of such parent.