Learn Hardcore Swedish: Familjen sitter i sitt hus. - The family is sitting in their house.

So Familjen sitter i sitt hus literally starts with The family. You normally need that definite form here, because you are talking about a specific family already known in the context, not just “a family.”

Why do we use sitter and not är (is/are)?

In Swedish, verbs like sitta (to sit), stå (to stand), ligga (to lie) are used much more than in English to describe where someone or something is located.

Familjen sitter i sitt hus.
= Literally The family sits in its house, but idiomatically often means The family is sitting in its house / is in its house (sitting).

You would only use är if you want to say The family is in its house without saying how they are positioned:

Familjen är i sitt hus. = The family is in its house. (no info about sitting/standing/etc.)

So sitter gives a bit more physical detail: they are there sitting, not just being there.

Does sitter here mean they are sitting right now, like an English continuous tense?

Yes, most naturally it refers to a present, ongoing situation, like English is sitting.

Swedish has only one present tense form (no separate “-ing” form), but it can correspond to both:

English sits (habitual / general)
English is sitting (right now)

Context decides which one is meant.

In this sentence, with a specific location i sitt hus, we normally understand it as a current situation: they are there right now, sitting in their house.

Why is it sitt hus and not sin hus or sina hus?

Sin / sitt / sina are the Swedish reflexive possessive pronouns meaning his/her/its/their own (referring back to the subject).

They change form depending on the grammatical gender and number of the possessed noun:

sin – for en-words (common gender), singular
- sin bil = his/her/their own car (car = en bil)
sitt – for ett-words (neuter), singular
- sitt hus = his/her/their own house (house = ett hus)
sina – for any plural noun
- sina hus = his/her/their own houses

Hus is a neuter noun (ett hus), so we must use sitt:
Familjen sitter i sitt hus.

Why does sitt agree with hus and not with familjen?

Swedish possessive words normally agree with the thing owned, not with the owner.

Owner (subject): familjen (common gender)
Thing owned (object): hus (neuter, ett)

Because hus is an ett-word in singular, the reflexive must be sitt:

Familjen sitter i sitt hus.
(the family – in its own house)

If the possessed noun changed, the form would change:

Familjen sitter i sin lägenhet.
(lägenhet = en lägenhet → use sin)
Familjen sitter i sina rum.
(rum = rooms, plural → use sina)

What is the difference between sitt hus and deras hus?

Both can translate to their house, but they differ in whose house is meant:

sitt hus – reflexive: the house belongs to the subject of the sentence.
- Familjen sitter i sitt hus.
  = The family is sitting in its own house.
deras hus – non‑reflexive: the house belongs to some other “them”, not the subject.
- Familjen sitter i deras hus.
  = The family is sitting in their house (someone else’s house — another family’s, for example).

So sitt always points back to the subject; deras does not.

Could this also mean “The family lives in its house”?

Not really. Sitter describes a physical sitting position and is normally about a current situation, not a permanent dwelling.

To say The family lives in its house, you would use:

Familjen bor i sitt hus.

So:

sitter → sitting (right now, or in a concrete scene)
bor → lives (resides, has its home)

Why is the preposition i used here? Could it be på?

i literally means in, and a house is treated as an inside space:

i huset = in the house
i sitt hus = in its (own) house

You would not normally say på sitt hus for being located inside the house; that would sound like being on top of the house, e.g.:

De sitter på huset. = They are sitting on the house (on the roof, for example).

So i sitt hus is the normal, idiomatic choice for being in the house.

Could you also say Familjen sitter hemma instead of i sitt hus?

Yes, but the nuance changes slightly:

Familjen sitter i sitt hus.
– Focus on the physical building (their house) as the location.
Familjen sitter hemma.
– Focus on the idea of being at home, not specifically mentioning the house as an object.

Both are correct; you choose depending on whether you want to emphasize the house or simply the state of being at home.

Why is hus not in the definite form (huset) here?

Definiteness is already expressed through the possessive (sitt). In Swedish, a possessed noun normally does not take the definite ending at the same time.

So you say:

sitt hus (not sitt huset)
mitt hus (my house), ditt hus (your house)
vårt hus (our house)

If you add both a possessive and a definite ending, it is usually incorrect or at least very marked.

How is familjen pronounced, especially the -jen part?

Approximate pronunciation in IPA: [faˈmɪljɛn]

fa- like fa in father, but a bit shorter
-mil- like English mill
-jen: the j is a y‑sound (as in yes), and -en sounds roughly like en in taken.

So it is roughly: fa-MIL-yen (with the stress on the second syllable).