Except as expressly provided in §§ 501 and 502 of this title, the duty to support a poor person unable to support himself/herself rests upon the spouse, parents, or children, in that order, subject to § 504 of this title as to expenses described therein. If the relation prior in order shall not be able, the next in order shall be liable, and several relations of the same order shall, if able, contribute according to their means.