Learn Hardcore German: Meine Geschwister und ich streiten uns selten, weil wir alle Humor haben. - My siblings and I rarely argue because we all have a sense of humour.

In German (and also in careful English), it’s considered more polite to mention yourself last in a list.
So Meine Geschwister und ich = my siblings and I, which sounds more natural than I and my siblings.

The case (Nominative) is the same either way, because the whole phrase is the subject of the verb streiten.

Why is it Geschwister? Does that mean “siblings”? Is it singular or plural?

Yes, Geschwister means siblings.

Geschwister is grammatically plural.
It refers collectively to brothers and/or sisters.
There is no normal everyday singular form like ein Geschwister in standard German (though das Geschwisterkind exists, it’s more technical/rare).

So Geschwister always comes with plural forms:

meine Geschwister (my siblings)
die Geschwister (the siblings)

Why is it Meine and not Mein in Meine Geschwister?

The possessive word must agree with the gender and number of the noun it belongs to.

Geschwister is plural.
The plural form of mein is meine.

So:

mein Bruder (masc. singular)
meine Schwester (fem. singular)
meine Geschwister (plural)

Therefore Meine Geschwister is correct.

Why is ich used and not mich in Meine Geschwister und ich?

Ich is the Nominative form (subject form) of the pronoun, and here it is part of the subject:

Wer streitet sich selten? – Meine Geschwister und ich.

Because it answers the “who?” as the subject, you must use ich, not mich (which is Accusative).

Why do we say streiten uns and not just streiten?

In German, sich streiten is typically used as a reflexive verb when people argue with each other:

wir streiten uns – we argue (with each other)
sie streiten sich – they argue

So:

streiten alone = to quarrel, to be in conflict (often more general, can be used with a direct object too)
sich streiten = to argue with one another; this is the usual way to express “we argue” in everyday speech when talking about people in conflict with each other.

Because the subject is wir, the reflexive pronoun becomes uns:

ich streite mich (rare; usually with “mit …”)
du streitest dich
er/sie/es streitet sich
wir streiten uns
ihr streitet euch
sie/Sie streiten sich

What exactly does uns mean here? Is it “each other”?

Literally, uns is “ourselves”, but in this context it has a reciprocal meaning: “each other”.

So wir streiten uns is usually understood as:

we argue with each other

You could also say wir streiten miteinander or wir streiten (uns) gegenseitig, but wir streiten uns is the most natural everyday expression.

Where should selten go in the sentence, and could it go somewhere else?

In the sentence:

Meine Geschwister und ich streiten uns selten …

selten (rarely, seldom) is an adverb of frequency and typically goes:

after the conjugated verb and its pronoun: streiten uns selten

You could also say:

Wir streiten uns selten, … (same meaning, slightly different focus)
Selten streiten wir uns, … (emphasizes how rare it is; a bit more stylistic)

But you would not normally split it awkwardly, e.g. wir selten uns streiten – that’s wrong.

Why is there a comma before weil, and why does the word order change after weil?

Weil is a subordinating conjunction (“because”). In German, such conjunctions start a subordinate clause, and that clause is separated by a comma.
In subordinate clauses, the finite verb (the conjugated verb) goes to the end of the clause.

So:

Main clause: Meine Geschwister und ich streiten uns selten, …
Subordinate clause: … weil wir alle Humor haben.

Note the verb position:

Main clause: wir haben Humor (verb in 2nd position)
Sub clause (with weil): weil wir alle Humor haben (verb at the end)

What’s the difference between weil and denn here? Could we say …, denn wir alle haben Humor?

Yes, you could say:

Meine Geschwister und ich streiten uns selten, denn wir alle haben Humor.

Differences:

weil = “because” (strong causal link, subordinating → verb at the end)
denn = “for / because” (gives a reason, coordinating → normal main-clause word order)

Word order:

…, weil wir alle Humor haben. (verb at the end)
…, denn wir alle haben Humor. (verb stays in 2nd position)

Both are correct; weil is more common in spoken German.

Why is the verb at the end in weil wir alle Humor haben?

Because weil introduces a subordinate clause. In German, in subordinate clauses:

the conjugated verb normally goes in final position.

So:

wir haben Humor (main clause)
weil wir Humor haben (subordinate clause → haben goes to the end)

Why do we say Humor haben and not humorvoll sind or something like “are funny”?

Humor haben is the normal idiomatic way to say someone has a sense of humor.

wir alle haben Humor = we all have a sense of humor.
wir sind alle lustig = we are all funny (we ourselves make people laugh).
humorvoll is more like “humorous” or “full of humor”; it sounds a bit more formal/literary: wir sind humorvoll is less common in everyday speech.

So Humor haben is the most natural phrase when you want to say “have a sense of humor,” not necessarily “are comedians.”

Why is it wir alle Humor haben and not something like wir Humor alle haben?

Alle normally comes before the noun or pronoun it refers to, or immediately after the pronoun in subject position:

Correct patterns:

wir alle haben Humor
alle wir haben Humor (possible but less common; more emphasis)
wir haben alle Humor (emphasizes “all” a bit differently; also correct)

wir Humor alle haben is not correct word order.

In the sentence:

weil wir alle Humor haben

the structure is:

wir alle (we all)
Humor haben (have humor)

Why are the verbs streiten and haben in the plural form?

Because the subject is plural:

Meine Geschwister und ich → together this is “we”.

So:

wir streiten (not ich streite or er streitet)
wir haben (not ich habe or er hat)

Conjugation must match the person and number of the subject, which here is first-person plural (wir).