Learn Arabic (Modern Standard): عائلتنا كبيرة.

In Arabic, possessive words like "my" or "our" are attached to the end of the noun as suffixes. Here, the suffix ـنا means "our". When attached to عائلة (family), it creates عائلتنا (our family).

The word for family is normally written as عائلة. Why does the last letter change in عائلتنا?

The word عائلة ends with a taa marbuuta (ة). When you attach a pronoun suffix like ـنا (our) to a word ending in ة, that letter "opens up" and is written as a regular ت.

Why does the word for big (كبيرة) end with a feminine marker?

Adjectives must match the gender of the noun they describe. Because عائلة (family) is a feminine noun, the adjective must also be feminine, so we add the taa marbuuta (ة) to make كبيرة instead of the masculine كبير.

Where is the word for "is" in this sentence?

Arabic doesn't use a word for "is" or "are" in the present tense. You simply state the subject (عائلتنا) followed by the describing word (كبيرة) to mean "Our family is big."