GG5 and 中辞典 point at 親類 for which they (as we) have singular glosses. ルミナス has singular glosses too.

I think I prefer to stay with the singular glosses here.

@@ -18,2 +18,2 @@
-<gloss>relatives</gloss>
-<gloss>family</gloss>
+<gloss>relative</gloss>
+<gloss>relation</gloss>

Relatives: 820  (while the singular has many matches, manually scrolling through entries, plural seems much more prevalent)
https://context.reverso.net/translation/japanese-english/親族#relatives

Family: 161
https://context.reverso.net/translation/japanese-english/親族#family

Kin: (120)
https://context.reverso.net/translation/japanese-english/親族#kin

eijiro:
https://eow.alc.co.jp/search?q=親族
blood kin
family（血族である）
kin
kindred〔【用法】複数扱い〕
kinsfolk〈米・古〉
relation
rellie〈豪俗〉〔【同】relative〕
rels〈豪俗〉〔【同】relative〕
sib〔集合的〕

daijs: blood or marriage...
１ 血縁関係・婚姻関係のある人々。親類。親戚 (しんせき) 。
２ 民法上、六親等内の血族および配偶者と、三親等内の姻族。

Undoing singular-form edit. (族 is exceptional)

Wikipedia xref is actually to "Kinship", or I probably wouldn't have thought to include "kin".  But eijiro also has it.  

I don't think "relation" really fits here (except as a longer "blood relation"), and it's practically absent on reverso:
https://context.reverso.net/translation/japanese-english/親族#relation

@@ -18,2 +18,3 @@
-<gloss>relative</gloss>
-<gloss>relation</gloss>
+<gloss>relatives</gloss>
+<gloss>family</gloss>
+<gloss>kin</gloss>

prog

--> singular form
Not [adj-no].

@@ -18,2 +18,2 @@
-<pos>&adj-no;</pos>
-<gloss>relatives</gloss>
+<gloss>relative</gloss>
+<gloss>relation</gloss>