attend vs sit in

attend and sit in both are verbs.

Word	Noun	Adjective	Verb	Adverb
attend	No	No	Yes	No
sit in	No	No	Yes	No

As verbs, sit in is a hyponym of attend; that is, sit in is a word with a more specific, narrower meaning than attend:

Other hyponyms of attend include worship.

attend (verb)	sit in (verb)
give heed (to)	participate in an act of civil disobedience
work for, or be a servant to	attend as a visitor
take charge of or deal with
be present at (meetings, church services, university), etc.
to accompany as a circumstance or follow as a result