Learn Hardcore Icelandic: Systkinið kemur í kvöld.

Plural definite would be systkinin (the siblings), with ending -in.
The bare form systkini can be either singular (a sibling) or plural (siblings), depending on context.

Why is the verb kemur (singular) used and not koma (plural)?

Because systkinið is grammatically singular. Icelandic verbs agree with grammatical number:

Singular: systkinið kemur …
Plural: systkinin koma …

How would I say the same sentence if I meant “the siblings” instead of “the sibling”?

Use the plural definite subject and plural verb: Systkinin koma í kvöld.

What exactly does í kvöld mean? Is it “this evening” or “tonight”?

Í kvöld means “this evening/tonight” (the evening of today). It’s used for events in the evening hours. For the night proper (late night/overnight), Icelandic uses í nótt.

Which case does kvöld take after í here?

Time expressions with í like í kvöld, í dag, í gær use the accusative. Kvöld is a neuter noun whose nominative and accusative forms are the same, so you don’t see a change.

Can I move í kvöld to the front? What happens to word order?

Yes. Icelandic is a V2 language in main clauses, so the finite verb stays in second position:

Neutral: Systkinið kemur í kvöld.
Fronted time: Í kvöld kemur systkinið.
Yes–no question: Kemur systkinið í kvöld?

How is the verb “come” conjugated around this sentence?

The verb is að koma.

Present: ég kem, þú kemur, hann/hún/það kemur; við komum, þið komið, þeir/þær/þau koma
Past (simple): ég/hann kom; við/þeir komu
Perfect: hefur/hafa komið
Future is typically expressed with context or with mun(a): hann mun koma

How do numbers work with systkini?

It’s neuter, so use neuter numerals:

eitt systkini (one sibling)
tvö systkini (two siblings)
þrjú systkini (three siblings) Plural definite for “the siblings” is systkinin.

How do I say “my sibling” or “my siblings” with this noun?

Systkinið mitt = my sibling (neuter singular; possessive after the noun)
Systkinin mín = my siblings (plural definite; possessive after the noun) You can also say mítt systkini / mín systkini in some contexts, but after-the-noun is most common with definite forms.

Is systkini a gender‑neutral word?

Yes. Systkini is a gender‑neutral term for a sibling. If you want gendered terms, use bróðir (brother) or systir (sister).

Pronunciation tips for this sentence?

Systkinið: stress on the first syllable. Both y and i sound like short “i” (as in English “bit”). Final ð is like the “th” in this, but very light at the end.
kemur: “keh-mur,” with a short e.
í: long “ee.”
kvöld: roughly “kveult.” The ö is like German ö/French eu. Final d is devoiced, sounding close to “t.”

Why is there no article before kvöld?

Because í kvöld is a fixed adverbial time expression meaning “tonight/this evening.” Icelandic doesn’t use an article in such time phrases.

Is it common to refer to someone simply as “the sibling” in Icelandic?

You’ll more often specify whose sibling, e.g. Systkinið mitt (my sibling) or use a name/kin term like bróðirinn/systirin if known. Systkinið by itself assumes a context where “the sibling” is already identifiable.