Both Uses of

relative
in
Arrowsmith

While the others drifted, often by chance, often through relatives, into their various classes, Martin remained doubtful.†
Chpt 10 *

relatives = people related by blood or marriage

They were married the following January, in St. George's Church, and Martin suffered almost as much over the flowers, the bishop, the relatives with high-pitched voices, and the top hat which Joyce had commanded, as he did over having Rippleton Holabird wring his hand with a look of, "At last, dear boy, you have come out of barbarism and become One of Us."†
Chpt 37

Definitions:

(1) (relative as in: the relative importance)
compared with something else (not an absolute value or not complete)
(2) (relative as in: they are relatives)
connected
in various senses, including:
- a person related by blood or marriage -- as in "The hospital won't let me visit her because I'm not a relative."
- a plant or animal related by origin or grouping -- "The closest relative of the dog is the gray wolf."
(3) (relative as in: questions relative to the topic)
related to
(4) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

More specialized and less common senses of the word are found in philosophy, grammar, and music. Consult a comprehensive dictionary if you wish to see those. All senses have to do with relationships.