Translation:My parents are bilingual, but I am not.
Irish pronouns ‘mé, t(h)ú, (s)é, (s)í, sınn/muıd, sıbh, sıad’ have the respective emphatic forms: ‘mıse, t(h)usa, (s)eısean, (s)ıse, sınne/muıdne, sıbhse, sıadsan’. The suffix that is used to form these (e.g. -se for mé, -sa for tú, etc.) can be added to conjugated forms of both verbs and prepositions to imply the same emphatic forms, however the vowel in these suffixes is changed to ‘a’ or ‘e’ depending on whether the predecing consonant is broad or slender. Hence, ‘liom’ (with me) yields ‘liomsa’, while ‘uaım’ (from me) yields ‘uaımse’, and similarily ‘nílım’ becomes ‘nílımse’. The third person masculine/plural suffix ‘-s(e)an’ is hyphenated if preceded by another ‘s’, hence ‘leis’ (with him) yields ‘leis-sean’.
Why would you expect American English textbooks to be definitive about anything, outside of whatever the textbook writer wants you to think? Even an American English dictionary like Merriam Webster contains a definition for "amn't" - https://www.merriam-webster.com/dictionary/amn%27t
It's always a good idea to check the dictionary before claiming that a word isn't a word.
Interestingly, it is suggested that this form fell out of favour in English because English speakers don't like to pronounce "m" and "n" together (so words like "column" end up with a silent "n"). But Irish likes epenthetic vowels, so "amn't" didn't pose the same problem for people that were familiar with speech patterns in Irish.
That's just leathan le leathan, caol le caol - if the word that you're adding the suffix to ends in broad letter, use the broad sa suffix, if it ends in a slender letter, use the slender se suffix. liomsa, tusa were broad (om and u endings), mise and nílimse were slender (i and im).
The -se vs. -sa choice exists for first-person singular, second-person singular, second-person plural, and third-person singular feminine forms.
For first-person plural forms, -e is always used.
For third-person singular masculine and third-person plural forms, the choice is between -sean and -san.