Would this be considered a science fiction? I have to write a science fiction about my pets. Is the science fiction? There are two dogs. One is Paco, the other is Gordita. Gordita loves Paco a lot and he loves her a lot and they are married. One day she accuses him of loving their owner, Sam, more than he loves her. She gets really mad and attacks Sam by biting into his neck. Sam dies, so Sam's mother kills Paco and Gordita.
Side note: they refer to their owners as their humans... as in, the humans are the pets, not the dogs.

No, that is not science fiction.

From Wikipedia --

- A time setting in the future, in alternative timelines, or in a historical past that contradicts known facts of history or the archaeological record.
- A spatial setting or scenes in outer space (e.g. spaceflight), on other worlds, or on subterranean earth.
- Characters that include aliens, mutants, androids, or humanoid robots.
- Technology that is futuristic (e.g. ray guns, teleportation machines, humanoid computers).
- Scientific principles that are new or that contradict known laws of nature, for example time travel, wormholes, or faster-than-light travel.
- New and different political or social systems (e.g. dystopia, post-scarcity, or a post-apocalyptic situation where organized society has collapsed).
- Paranormal abilities such as mind control, telepathy, telekinesis, and teleportation.
- Other universes or dimensions and travel between them.

Wouldn't it be a new social system though?

Nope, not the way you described it. You give no backstory or background, and what you wrote is confusing and leaves too much for the reader to puzzle out.

How many words does have to be?

I would call it fantasy, as it involves animals reasoning and talking to each other. You could make it a SciFi story with a few minor changes - perhaps the owner injects some sort of experimental drug into his pets which enhances their brain functions so that they gain the ability to reason, communicate, and experience love and jealosy (sort of like what happens in "Rise of the Planert of the Apes.")