Learn Hardcore Danish: Min ven er altid venlig mod sin familie. - My friend is always kind to his/her family.

sin / sit / sine – reflexive possessives (referring back to the subject of the same clause)
hans / hendes / dens / deres – non‑reflexive possessives (someone else’s, or at least not clearly the subject’s)

In Min ven er altid venlig mod sin familie, the subject is min ven (my friend).
Sin refers back to that subject, so the sentence means:

My friend is always kind towards his own family.

If you said:

Min ven er altid venlig mod hans familie.

it would normally mean:

My friend is always kind towards his (another man’s) family, not his own.

So sin is used because the family belongs to the subject min ven.

Why is it sin familie and not sine familie?

The form of the reflexive possessive agrees with the grammatical number and gender of the noun, not with the owner:

sin – common gender, singular (en‑words, singular)
sit – neuter, singular (et‑words, singular)
sine – all plurals (both en‑ and et‑words in plural)

Familie is an en‑word: en familie, and here it is singular.
Therefore the correct form is:

sin familie (one family)

You would use sine with a plural noun:

sin familie – his/her (own) family (one family)
sine børn – his/her (own) children (plural noun)

Why isn’t there an article before familie? Why not sin familien?

Danish uses the definite form of nouns very differently from English. After possessive pronouns (like min, din, hans, sin, vores), you normally use the indefinite noun form:

min ven – my friend
din bog – your book
hans bil – his car
sin familie – his/her own family

You normally do not say:

✗ min vennen
✗ sin familien

So sin familie is the correct structure: possessive + indefinite noun.

Why is venlig not venlige here?

The form venlig is used because the adjective is predicative, coming after the verb er and describing the subject, not directly in front of the noun:

Min ven er venlig. – My friend is kind. (predicative)
en venlig ven – a kind friend (attributive)

For predicative adjectives (after er, bliver, etc.), Danish most often uses the base form for singular subjects:

Han er træt. – He is tired.
Hun er glad. – She is happy.
Min ven er venlig.

You get venlige as an attributive plural or definite form:

De er venlige. – They are kind. (plural subject)
de venlige venner – the kind friends (definite, plural)

Could I say Min ven er altid flink mod sin familie instead of venlig?

Yes, but there is a nuance difference:

venlig – kind, polite, friendly in behavior and attitude; a bit more neutral/formal
flink – nice, friendly, often used in more everyday, informal speech; can also mean “well‑behaved” (especially about children)

Min ven er altid venlig mod sin familie highlights that he is consistently kind and polite toward them.
Min ven er altid flink mod sin familie feels slightly more informal and can sound like “he’s always nice/good to his family.” Both are natural, but venlig is a bit more neutral/formal.

Why is the word order er altid venlig and not er venlig altid?

Danish has fairly strict rules for adverb placement. Adverbs like altid, ofte, aldrig usually go in “middle position” – that is, after the finite verb but before most other elements in a simple main clause:

Jeg er altid træt om morgenen.
Han kommer ofte for sent.
Vi spiser aldrig kød.

So:

Min ven er altid venlig mod sin familie
(subject – verb – adverb – adjective – rest)

Er venlig altid would sound strange or wrong in normal Danish. You can move altid to emphasize it in some contexts, but the default neutral placement is after er.

Why is it Min ven and not Min ven er with some verb before the subject, like in German?

In a neutral statement in Danish main clauses, the typical word order is:

Subject – Verb – (Adverb) – Rest

So:

Min ven (subject)
er (verb)
altid (adverb)
venlig mod sin familie (rest)

You only move the verb before the subject (V2 rule with inversion) when something else is placed first in the sentence:

I dag er min ven altid venlig mod sin familie.
(Adverbial fronted → verb er comes before subject min ven)

But in your sentence there is nothing before the subject, so Min ven er … is the normal order.

Is ven always masculine, like “male friend”? What if it’s a female friend?

Ven is grammatically common gender (en‑word), but its natural gender is usually male.
For a female friend, Danish often uses:

veninde – female friend

So:

Min ven er altid venlig mod sin familie. – My (male) friend …
Min veninde er altid venlig mod sin familie. – My (female) friend …

In some modern or gender‑neutral contexts people might use ven more generically, but the traditional distinction is:

ven (male), veninde (female).

Can sin ever refer to someone other than the subject, like in English “his”?

In standard Danish, sin / sit / sine almost always refer back to the grammatical subject of the same clause. That’s their main function, and their use is much stricter than English his/her/their.

Compare:

Peter siger, at han elsker sin familie.
→ Peter says that he loves his own family (the subject of the clause with elsker is han, so sin refers to han).

If you want to clearly talk about someone else’s family, you switch to hans/hendes/deres:

Peter siger, at han elsker hans familie.
→ Normally understood as: Peter says that he (some other man) loves his family.

Because of this, learners need to remember:
If the owner is the subject of the clause, and it’s a 3rd‑person owner (he/she/they), sin/sit/sine is usually the right choice.