jan Alanto o, toki!
Unfortunately, Sonja did not create very exactly the word list at the end of the official Toki Pona book. For example she lists "tan" as preposition only but she use it as verb also: "sina tan ala tan ma Tosi?" (pu page 64).
The same with "kepeken". She use it often as verb: "jan pali li kepeken ilo." (pu page 64, btw: kepeken can a noun here also) "o kepeken ala ilo ike." (pu page 65, btw: kepeken can a noun here also).
Other word lists are better, for example http://tokipona.net/tp/ClassicWordList.aspx
"tan" is list here as noun also.
In this amount I have explained that the first slot in a "la" phrase is mostly for a noun or pronoun: viewtopic.php?f=5&t=2659#p15705
With this knowledge you can analyze your sentence as follows:
tan jan seme la sina pu? - noun adjective seme as adjective la ...
(Which is the human cause ...?)
OK, you think "tan" can be a preposition here also. But how can we distinguish whether it is a noun or preposition? I can't see a solution. The make it clear is the best to put the preposition on the right place in the verb phrase at the end of the sentence.
Consider another example:
mi moku e moku, kepeken ilo moku li moku e telo, kepeken poki.
This sentence has two verb phrases with a prepositional object each: "moku e moku, kepeken ilo moku" and "moku e telo, kepeken poki" ("kepeken" is a preposition in each verb phrases).
If you put one of this prepositional object in a "la" phrase you will break up the corresponding verb phrase. You can't see to wich verb phrase is the prepostion connected to.
This example is little different:
mi moku e moku, kepeken ilo moku li moku e telo, kepeken poki, lon tenpo ni.
The prepositional object "lon tenpo ni" is part of the second verb phrase. But If you mean "tenpo ni" for both verb phrase you have to put it in a "la" phrase:
tenpo ni la mi moku e moku, kepeken ilo moku li moku e telo, kepeken poki.
O sorry: "tenpo" is a noun here
In your example "kepeken ilo sitelen la jan li ken sitelen lon lipu." is "kepeken" possible as noun (use, usage).
In the example "kulupu pali li kepeken seme?" "kepeken" can be a verb or a noun.
Btw: After a verb can't be an adjectives. To modify verbs you use adverbs.