The answer is
SISTER
Because, as others have pointed out:
The puzzle hints at a fourth stanza made up of words formed by following the pattern for each parallel set of words in the first three.
The missing stanza, borrowing heavily from Alconja but with some key corrections, is:
Look at me I am a nun
I am the aunt of a son
I sound like ə after a cyst
Mixed up I am the word resist
The final solution:
A nun is a sister.
If you have a son, your sister is his aunt. — Thanks @Gareth!
Phonetically, "cyst" + ə (actually ə r) would sound like sister.
And sister is an anagram of resist
The gory details
First line:
$\small\begin{array}{lll} \phantom{XXXXXXXXXXXXXXXX}\\\text{- Boot,Book,Loot → Look}&&\text{first letter doubles, b/b/l/l; last letter alternates t/k/t/k}\\ \text{- percentage,dollar,pound → at}&&\text{names of top row symbols on QWERTY,}\\ &&\ \ \ \ \text{in order backward from shift-5 to shift-2}\\ \text{- jee,kee,lee → mee⇒me}&&\text{{j/k/l/m letter run}+“ee”, then made into a word}\\ \text{- afternoon,morning,pm → am}&&\text{afternoon:morning :: pm:am}\\ \text{- dee,see,bee → a}&&\text{d/c/b/a letter run}\\ \text{- few,couple,one → none⇒nun}&&\text{3/2/1/0 sequence as words, then homophoned}\end{array}$
Second line:
$\small\begin{array}{lll} \phantom{XXXXXXXXXXXXXXXX}\\\text{- Mouth,Nose,Ear → Eye⇒I}&&\text{parts of the face, then homophoned}\\ \text{- ohm,I'm,em → am}&&\text{{o/i/e/a (reverse) vowel sequence}+“m”}\\ \text{- thenceforth,thence,then → the}&&\text{incremental truncations that still form words}\\ \text{- mother,father,uncle → aunt}&&\text{male and female words for types of relatives}\\&&\ \ \ \ \text{(better might be father:mother :: uncle:aunt)}\\ \text{- offends,offend,off → of}&&\text{incremental truncations that still form words}\\ \text{- see,bee,flat-bee → a}&&\text{sequence of musical half-tones, C, B, B♭, A}\\ \text{- Earth,Venus,Mercury → Sun⇒son}&&\text{Solar System from Earth inward, then homophoned}\end{array}$
Third line:
$\small\begin{array}{lll} \phantom{XXXXXXXXXXXXXXXX}\\\text{- Tee,Why,You → I}&&\text{(homophones for) 2nd row QWERTY letters t/y/u/i}\\&&\ \ \ \ \text{(previously was “Why,You,Oh”, an error)}\\ \text{- taste,smell,sight → sound}&&\text{senses. (It would have been more symmetric if the senses}\\&&\ \ \ \ \text{were aligned with the parts of the face from line 2,}\\&&\ \ \ \ \text{but it certainly isn't a flaw that they're not)}\\ \text{- lack,lake,lick → like}&&\text{“l”+{alternating short vowel/long vowel sounds, ă/ā/ĭ/ī}}\\&&\ \ \ \ \text{+{alternating ck/ke/ck/ke}}\\ \text{- ɘ,ɚ,ɵ → ə}&&\text{‘e’-like IPA (pronunciation) symbols}\\ \text{- pre,post,before → after}&&\text{pre:post :: before:after}\\ \text{- eff,dee,es → a}&&\text{home row QWERTY letters in backward order f/d/s/a}\\&&\ \ \ \ \text{(note: “es” s/b “ess”? ...was originally “see”, an error)}\\ \text{- gist,fist,dist → sist⇒cyst}&&\text{{home row QWERTY letters in backward order g/f/d/s}}\\&&\ \ \ \ \text{+“ist”, then made into a word}\end{array}$
Fourth line:
$\small\begin{array}{lll} \phantom{XXXXXXXXXXXXXXXX}\\\text{- Mix,Mixes,Mixing → Mixed}&&\text{tenses of “to mix”}\\ \text{- right,left,down → up}&&\text{directional words; right:left :: down:up}\\ \text{- he,she,you → I}&&\text{singular pronouns, progressing from}\\&&\ \ \ \ \text{3rd person to 2nd person to 1st person}\\ \text{- is,is,are → am}&&\text{conjugation of “to be” for the preceding pronoun}\\ \text{- a,the,a → the}&&\text{articles, alternating between a/the/a/the}\\ \text{- digit,number,letter → word}&&\text{digit:number :: letter:word}\\ \text{- desists,resists,desist → resist}&&\text{first letter alternates d/r/d/r;}\\&&\ \ \ \ \text{first two are inflected, second two are not s/s/ / }\end{array}$
