Both Uses of

relative
in
The Phantom Tollbooth

"For I found him," continued the doctor, ignoring this outburst, "living alone and unwanted in an abandoned soda bottle—without family or relatives—"†
p. 139.9 *

relatives = people related by blood or marriage

he replied, lowering his arm to the vast relief of the bug: that is, comparatively no. What I mean is, relatively maybe—in other words, roughly perhaps.†
p. 221.4 *

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.