Both Uses of

relative
in
The Killer Angels

M. Forster "With all my devotion to the Union and the feeling of loyalty and duty of an American citizen, I have not been able to raise my hand against my relatives, my children, my home.†
Chpt Frwd. *

relatives = people related by blood or marriage

Lee was watching the fight, which was now relatively quiet.†
Chpt 1.4 *

relatively = in comparison to something else

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.