poor relation vs replacement

poor relation and replacement both are nouns.

Word	Noun	Adjective	Verb	Adverb
poor relation	Yes	No	No	No
replacement	Yes	No	No	No

As nouns, replacement is a hypernym of poor relation; that is, replacement is a word with a broader meaning than poor relation:

Other hypernyms of poor relation include substitute.

poor relation (noun)	replacement (noun)
a person or thing considered inferior to another or others.	the act of furnishing an equivalent person or thing in the place of another
	a person or thing that takes or can take the place of another
	an event in which one thing is substituted for another
	a person who follows next in order
	someone who takes the place of another person
	filling again by supplying what has been used up