Both Uses of

relative
in
Brave New World

Lenina nodded her averted head, let out the breath she had been holding and managed to draw another one, relatively untainted.†
p. 121.8 *

relatively = in comparison to something else

But if we happen to be Alphas, our bottles are, relatively speaking, enormous.†
p. 223.1

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.