Learn Hardcore Swedish: Han och hennes syster är nästan i samma ålder. - He and her sister are almost the same age.

In the sentence Han och hennes syster är nästan i samma ålder, the subject of the verb är is the whole phrase han och hennes syster. Since han is part of the subject, you must use the subject form han, not honom.

Compare:

Han är trött. – He is tired. (subject → han)
Jag såg honom. – I saw him. (object → honom)

Why is it hennes syster and not sin syster?

Both hennes syster and sin syster can mean her sister, but they work differently:

hennes syster = her sister (refers to some female person, not necessarily the subject)
sin syster = her own sister (or his/its own sister), and it normally refers back to the subject of the clause

In this sentence, han (he) is the subject, and the sister belongs to some other woman, not to han. So you cannot use sin; sin would refer back to han, which would mean his own sister.

So:

Han och hennes syster = He and her sister (the sister belongs to another woman)
Han och hans syster = He and his sister (the sister belongs to him)
Hon och sin syster would usually be understood as She and her own sister

Can hennes change form depending on the noun’s gender or number?

No. Hennes never changes. It is invariable.

Whether the noun is common or neuter, singular or plural, you still use hennes:

hennes syster – her sister
hennes barn – her child / her children
hennes bok – her book
hennes hus – her house

Unlike min / mitt / mina or sin / sitt / sina, which change with gender and number, hennes stays the same.

Why is it är and not har to talk about age?

Swedish uses är (to be) for age, just like English, not har (to have) like some other European languages.

Examples:

Han är 20 år. – He is 20 years old.
De är nästan i samma ålder. – They are almost the same age.

Using har here would be incorrect:

✗ Han har 20 år. – wrong in Swedish.

What does nästan mean, and why is it placed before i samma ålder?

Nästan means almost or nearly. In this sentence it modifies the whole phrase i samma ålder (in the same age).

Word for word:

är i samma ålder – are in the same age / are the same age
är nästan i samma ålder – are almost the same age

Placing nästan before the phrase it modifies is normal in Swedish. You could also see it in other contexts:

Hon är nästan klar. – She is almost finished.
De bor nästan grannar. – They live almost as neighbours / practically next door.

Why do you say i samma ålder and not something like av samma ålder?

The normal Swedish preposition for this expression is i:

i samma ålder – (in) the same age → the same age

You do not say av samma ålder in this meaning. The pattern is:

De är i samma ålder. – They are the same age.
Barn i samma ålder leker ofta ihop. – Children of the same age often play together.

The preposition i is used metaphorically, like “in the same age group”.

Could you also say lika gamla instead of i samma ålder?

Yes. Another very natural way to say it is:

Han och hennes syster är nästan lika gamla. – He and her sister are almost the same age.

Lika gamla literally means equally old. Both versions are common:

är i samma ålder
är lika gamla

They are close in meaning; lika gamla sounds slightly more direct and everyday.

Why is it samma ålder and not den samma ålder or samma åldern?

In this fixed expression, samma works like a determiner and already has a “definite” feel, so you do not add the definite ending -n or den:

samma ålder – the same age (correct)
✗ samma åldern – wrong
✗ den samma ålder – wrong

This is similar in other phrases:

i samma stad – in the same city
på samma skola – at the same school

You only say densamma as a separate pronoun, e.g. Det är fortfarande densamma. – It is still the same.

Can I switch the order and say Hennes syster och han är nästan i samma ålder?

Yes, that is grammatically correct:

Han och hennes syster är nästan i samma ålder.
Hennes syster och han är nästan i samma ålder.

Both are fine. In everyday speech, people often put han or hon first if that person is the main focus, but Swedish does not have a strict rule about the order in this kind of simple coordination.

Is ålder an en-word or ett-word, and how does it inflect?

Ålder is an en-word (common gender).

Key forms:

singular indefinite: en ålder – an age
singular definite: åldern – the age
plural indefinite: åldrar – ages
plural definite: åldrarna – the ages

Examples:

Vid hög ålder – at an advanced age
I min ålder – at my age
i samma ålder – (in) the same age

Why not say nästan samma ålder instead of nästan i samma ålder?

You can say nästan samma ålder, but then the structure usually changes a bit:

De har nästan samma ålder. – They have almost the same age. (possible, but less idiomatic)
De är nästan i samma ålder. – They are almost the same age. (more natural)

With är, the fixed expression is (vara) i samma ålder.
With ha, you can say ha samma ålder, but this is less common than simply:

De är (nästan) lika gamla.
De är (nästan) i samma ålder.

So the original sentence sounds very natural and idiomatic.

Is the sentence talking about two different people or could hennes syster be his own sister?

In normal interpretation, hennes syster refers to some other woman’s sister, not his.

Han och hennes syster → He and her sister (her = another female)
If you wanted to say he and his (own) sister, you would usually say:
Han och hans syster – He and his sister

Because hennes can only mean her (a female person, not his), most listeners will understand that the sister belongs to a different woman, not to han.