Learn Finnish: Hän vaatii kotirauhaa perheelleen. - He demands domestic peace for his family.

In Finnish, when a third-person subject (here, hän) is doing something for their own possession (his own family), you usually omit the genitive pronoun hänen. Using hänen perheelleen in this sentence would actually imply he is demanding peace for someone else's family! The reflexive ownership (his own family) is shown solely by the possessive suffix at the end of perheelleen.

Where does the -en at the end of perheelleen come from? I thought the 3rd person suffix was -nsa or -nsä.

The basic 3rd-person possessive suffix is indeed -nsa/-nsä. However, when a word already has a case ending that finishes in a short vowel (like the allative -lle in perheelle), Finnish usually forms the possessive by lengthening that final vowel and adding -n. So, perheelle + possessive becomes perheelleen. You could technically say perheellensä, but perheelleen is much more common and flows better.

Why is the object kotirauhaa in the partitive case instead of the accusative?

The verb vaatia (to demand) takes the partitive case when the object is an abstract concept, an uncountable mass, or an ongoing process. Since "domestic peace" (kotirauha) is an abstract, unquantifiable state rather than a single, completed physical object, it takes the partitive ending -a.