Learn Polish: Kocisko mojej siostry śpi cały dzień. - My sister's big cat sleeps all day.

It is formed by taking the root of kot and adding the suffix -isko. Notice the consonant mutation where the t changes to c. While augmentatives can sometimes be pejorative, for animals it often carries an affectionate tone, painting a picture of a large, cuddly, or lazy pet.

Kot is a masculine noun. Does kocisko keep the same gender?

No, augmentatives ending in -isko or -sko are grammatically neuter. This means that any adjectives or pronouns describing the cat would also need to be in the neuter form, such as to wielkie kocisko (that big cat).

Why do we use mojej siostry instead of the dictionary form moja siostra?

This is the genitive case used to show possession, meaning 'of my sister'. In Polish, the noun in the genitive usually directly follows the noun it owns, which is why mojej siostry comes right after kocisko.

What case is cały dzień in this sentence?

It is in the accusative case, which is often used in Polish to express the duration of time (answering 'for how long?'). Because dzień is a masculine inanimate noun, its accusative form looks exactly the same as its nominative dictionary form.