I live with a couple that keeps threatening me to be kicked out. I am all the way from Michigan and now live in Seattle, WA. All my possessions are in their attic. What are my rights if they do kick me out??

No roommates, landlords, or building managers can throw out your possessions under the law, until a formal eviction is final, and even then there is often a statute about not throwing them out. Doesn't mean they won't, however, so tell us the story. You also sort of mix possessions with you yourself (being evicted?), so clarify that too.
I'm not going to look up the laws until you do.

Are you a minor?

Are you working?

Are you behind in rent?

Why do they want to kick you out?

The most obvious seems to simply move.

You better make arrangement for your stuff, since you now live in one place and your stuff is in another. If they are getting no rent from you, then after a while is it not abandoned? When do you intend to return? So far they just threaten, but unless you make your intentions clear, they can take more decisive and lawful actions to get rid of you, and stuff.

You need to act, unless there is a lot more to your story. At least heed their WARNINGS.

Are you paying rent, are you still living in their home? If you have moved away and merely left your belongings, they do not have to store them forever and may dispose of them after giving you proper time and warning to pick them up.

The couple are your landlords. Are they in Michigan and you have moved from there to Seattle? Do you expect that they would have to keep you stuff for you forever, or what?

You are not being clear. Did you move from Michigan leaving stuff in your former landlord's attic? Or did you move to Seattle and are storing your stuff in your new landlord's attic.

They cannot just throw you or your stuff out. They must obtain a valid court order to do so.

But why do they keep threatening?

... They cannot just throw you or your stuff out. They must obtain a valid court order to do so.

But why do they keep threatening?

If they gave OP written notice that, after a specified time (as required by law), they would throw the stuff out, and if OP then failed to make arrangements for disposition of the stuff (pick it up, etc.), I expect that they could throw it out. Perhaps that's what OP means by "threatening".