Questions & Answers about Их братья и мужья здесь.
Why do братья and мужья end in -ья instead of the usual plural endings like -ы or -и?
Брат (brother) and муж (husband) are part of a special group of masculine nouns that have irregular plurals. Instead of adding -ы or -и, they add a soft sign plus я (-ья). You just have to memorize these common exceptions, which also include words like друзья (friends) and стулья (chairs).
How do you pronounce the -ья ending in братья and мужья?
The soft sign (ь) tells you to keep a distinct "y" sound before the vowel. So, братья is pronounced like "brat-ya" (not "bra-tya"). It creates a tiny separation between the consonant and the "ya" sound.
Why doesn't the word for "their" (их) change to match the plural nouns, like мои (my) does?
Unlike words like мой (my) or наш (our) that change to match the gender and number of the noun they describe, the third-person possessives—его (his), её (her), and их (their)—are frozen. They never change their spelling or endings, no matter what noun comes after them.
Where is the word for "are" in this sentence?