Han er pæn mod sin søster.

Here pæn means something like nice, kind, or pleasant.

On its own, pæn often means pretty, good-looking, neat, or well-behaved, depending on context. But in the pattern pæn mod nogen, it usually means nice to someone or kind toward someone.

So Han er pæn mod sin søster means he behaves nicely toward his sister.

Mod is the preposition that means toward or towards.

In Danish, at være pæn mod nogen is the natural way to say to be nice to someone. English uses to, but Danish uses mod here.

So:

• pæn mod sin søster = nice to his sister
• literally: nice toward his sister

Danish uses sin/sit/sine when the possessor is the same as the subject of the sentence.

In this sentence:

• Han = the subject
• sin søster = his own sister

So Han er pæn mod sin søster means He is nice to his own sister.

If you said Han er pæn mod hans søster, it would normally mean He is nice to his sister, where his refers to some other male person, not the subject han.

This is one of the most important differences between Danish and English possessives.

The form depends on the noun being possessed:

• sin for common gender singular nouns
• sit for neuter singular nouns
• sine for plural nouns

Here, søster is a common gender singular noun, so you use sin.

Examples:

• sin søster = his/her own sister
• sit hus = his/her own house
• sine bøger = his/her own books

Because pæn is being used as a predicate adjective after er.

In Danish, adjectives often change form when they come before a noun, but after være they usually stay in their basic form unless grammar requires agreement.

Here:

• Han er pæn = He is nice / handsome / neat

Since han is singular and common gender, the basic form pæn is correct.

Compare:

• en pæn dreng = a nice boy
• et pænt barn = a nice child
• drengen er pæn = the boy is nice
• barnet er pænt = the child is nice

So predicate adjectives can still agree in some cases, but here han takes pæn.

Grammatically yes, pæn can mean good-looking or handsome, but in this sentence that is probably not the intended meaning because of mod sin søster.

The phrase pæn mod strongly suggests behavior, not appearance. So a learner should understand it as:

• He is nice to his sister

not:

• He is handsome toward his sister

That would not make sense in English either.

Yes, you often can.

• pæn mod nogen = nice / proper / good toward someone
• sød mod nogen = sweet / kind toward someone

Both are possible, but they are not always identical in tone.

Pæn can suggest being well-behaved, decent, or properly nice.
Sød sounds warmer and more affectionate, like sweet.

So:

• Han er pæn mod sin søster = He is nice to his sister
• Han er sød mod sin søster = He is sweet to his sister

No, this is normal Danish main-clause word order:

• Han = subject
• er = verb
• pæn = adjective
• mod sin søster = prepositional phrase

So the structure is:

Subject + verb + complement + prepositional phrase

This is a very basic and common sentence pattern in Danish.

If you made it a question, the verb would come first:

• Er han pæn mod sin søster? = Is he nice to his sister?

Usually it means his sister, and in many contexts that will be understood as a biological sister, half-sister, adoptive sister, or sister in the normal family sense.

Like English sister, the exact relationship depends on context. The grammar point is simply that sin shows the sister belongs to the subject.

So the important thing for a learner is:

• sin søster = his own sister / her own sister, depending on the subject

You would change er to var:

• Han var pæn mod sin søster.

That means:

• He was nice to his sister.

The rest of the sentence stays the same.