Learn Icelandic: Bæði systkinin voru heima í dag. - Both siblings were at home today.

Bæði is the neuter plural form. The pronoun has to match the gender of the noun it describes. Since systkin (siblings) is a neuter plural word, we must use bæði.

Why does the noun have the definite article attached (systkinin) when English just says "both siblings"?

In Icelandic, the word for "both" almost always pairs with the definite form of the noun. So you literally say "both the siblings" (bæði systkinin), "both the cars" (báðir bílarnir), and so on.

Why is Bæði systkinin in the nominative case here rather than the accusative?

It is in the nominative case because "both siblings" is the subject of the sentence (the ones performing the action of being at home). If they were the object of an action, like if you saw them, you would use the accusative form, which conveniently also happens to be bæði for neuter words!

As a quick refresher, why do we use heima instead of heim?

Heima means being "at home" (stationary), which fits perfectly with the verb voru (were). You only use heim when there is a destination or movement involved, like walking or driving home.