My mother passed away and a sister that was living with her thinks that everything that was my Mother's is hers. Any advice?

I'm not seeing any anger, just a lot of disagreement.
It would be nice if the OP had come back to clarify things because we are all just guessing at the details.

My comment was not intended to anger anyone, but of course, it did.

ShakingDustOff - interesting to see you've returned to the forum.  I remember you.  
My comment was not aimed at you, but of course, you're free to believe it was, if that's what you want to do.  

Y'all have fun with this one. I'm out.

I agree that it doesn't really matter if sis was a caregiver or not given the negligible value of the belongings, but I don't think it's a foregone conclusion that she was. The mom may not have been sick a day in her life. The mom and daughter may have been living together just for companionship or to share expenses. The OP did not share these details so we don't know.

huhjustme, so sorry for your Mother's passing. Has your Mother's estate been through County/City Probate yet? Depending on the net worth of the estate, either with a Will or without a Will, some State laws require the estate to go through Probate. Thus, nothing can be distributed. Everything sits in limbo. Some States can allow a house or a car to be sold at fair market value and the money goes to the Estate.

The County/City will appoint a family member to be the Administrator of the Estate. If Mom had money in the bank, then that account needs to change over to an Estate account, that way the Administrator can write checks to pay for bills. All the bills and copies of the checks need to be kept as the Court will request these items.

Now, if Mom was renting her home, then your sister would have needed to remove all of those belongings and store them somewhere. So I was wondering if there was a misunderstanding why that sisters may have those items at her home?

Whether the sister was a caregiver of a freeloader doesn't alter the essential part of my post, suing over household contents and a car (unless it is a fairly new high end one) may not be worth the effort. Nor is any mention made of the need to contest the ownership of the house itself or of a desire for any sentimental pieces, which would probably change my opinion.

I agree with the previous comment that it's unlikely these items are worth much money, so it leads me to ask: Do these items have sentimental value, monetary value, or is it more a case of just not wanting the sister to have the items (regardless of monetary or sentimental value)? Grief does terrible things - there is an estranged member of our family who was (still is?) seething about not getting a particular piece of furniture! Nevermind that the piece of furniture is essentially worthless to begin with and also has significant water damage. At this point, its monetary value is next to nothing. Sentimental value may or may not be affected by the water damage, but I'd assume it would be difficult to see an emotionally significant item that now has damage to it.
That leaves the 3rd possibility - she just does not want anyone else to have it and she's mad. Such a shame.

Anyway, please ask yourself why it's bothering you so much that your sister is taking these items. Once you have that answer, you'll know what to do.

This could be my sister asking this (although my mother is still alive). Mom and I own her house as joint tenants with right of survivorship; the contents of the house are mostly of the type the OP described. Even if I didn't plan to use most of the stuff when I take over the house, the effort and expense of liquidating it is pretty daunting, and I doubt there would be enough in the way of proceeds to make it worthwhile. I'm also the executor of Mom's will, so I will be legally required to account in some way for Mom's assets, but I'm really dreading it, especially the expectation of one sibling that there will be at least some kind of inheritance when my mother dies.

That was my thought as well, JoAnn. I think there may be some posts kind of jumping to conclusions here. There is nothing that says the sister was caring for their mother while others did nothing.

Let's allow the OP to come back (if they will) and explain a little further before we go off the deep end here.

I see nothing that says sister cared for Mom. Just that they were living together. Did Mom rent or own? This happened in my Dads family. My Aunt came home to live with 3 kids. Did she care for Mom, not really but she was there. When the house was sold the money went to Moms care. Aunt found a place to live and was allowed to take what she needed to set up a place to live. Do you all really need that "stuff". Maybe you each can ask for something of Moms and let your sister have what she needs to live.

Does the house belong to the sister? If so, unless otherwise stated in writing by your mother, so do the contents.

As DeeAnna asked, was your sister paid for the care she gave? And are you certain that she didn't contribute to the purchase of anything in the house?
A used car and furnishings are not going to be worth very much, in fact when my mom got rid of her home we discovered that it would cost more to host an auction than we would likely get for her things. Suppose you got $20,000 for her things, divided by 8 (I'm including the possibility of children from your deceased sib) you would each end up with $2500, less legal fees. Is it worth the fight?

Unfortunately your sister thinks that she is "OWED" everything in your Mother's house because SHE was the one who took care of your Mother while the rest of you just sat back and did nothing. While that reasoning does not seem fair to you, it seems fair to her.

Did your sister get reimbursed or paid for taking care of your Mother? Or did she have to give up her job and her house and her way of living to take care of your Mother? If your sister had to move from her own house into your Mother's house, what did she do with her furnishings? Did she put her living room and dining room furniture in storage or is it in your Mother's house? Did your sister have to sell her washer and dryer when she moved in with your Mother?

Before you get all huffy and go to court, all of the children need to have a family conference. If you can't agree about your Mother's belongings, then I suggest that you contact a MEDIATOR to mediate an agreement between the children before you go to the expense of hiring a lawyer.

Google "mediator for state of _____________"

Nebraska has the "Office of Dispute Resolution" to "enhance and to advance the use of mediation and Alternative Dispute Resolution (ADR) in courts and communities by partnering with ODR-approved mediation centers and others involved with ADR...is a helpful problem solving process that empowers disputing parties to prioritize and express their wants and needs in order to arrive at a mutual agreement." https://supremecourt.nebraska.gov/programs-services/mediation

What does her will state? If she died intestate, then your state/provincial laws come into play.