How can I prevent my sister from taking all my mom's assets?

If your sister files for guardianship and received it your Mom should have gotten paperwork to that effect. She is told of the hearing so she can be present. You as an interested party and any other siblings should be there too. This way you can contest the guardianship based on sister does not have Moms conerns at heart. If sister is given guardianship, any money she makes off of the sale of the house goes into a bank acct in Moms name. The state keeps an eye on guardians.

POA, sister could not get it without Mom assigning her. Mom would need to be present to sign the document in front of a lawyer and a notary. Even as a POA sister is not entitled to Moms money. That should go into a bank acct for her care only.

You need to contact APS for no other reason than to get the theft on record. If Medicaid is ever involved u will need to prove theft was involved.

You don't file for POA, so something is wrong with that right off the bat. POA is given to you by the person asking you to act as his/her POA. That person must be competent at the time. As POA you may act only as directed unless during your service as POA the person becomes to demented to act for is or herself. At that point the POA is a FIDUCIARY, a legal representative who MUST keep meticulous records, signing and selling and etc only as POA for the person. At any time this POA can be reported for fraud and will be investigated; will face jail if they enrich themselves with sale of anything.

I do not know when all of this occurred, but any sale of property belonging to your Mother and in her name on title/deed, is not legal. Open a case by reporting this to APS.
Your sister could have "filed" for guardianship. But not for POA. So check on the facts of all this through APS. Something is missing and askew in this story.

You need a lawyer! Get one!! POA oversees paying bills etc. not to sell everything and pocket the money.

Talk to federal Trade Commission this sounds Like identity theft . You don't file for POA - your mother has to be present and have witnesses present and this POA would have to be Notarized so the sale of the house was Illegal . I doubt your sister had any authority to sell her house . What she did was theft and could go to Jail this is what we call Elder abuse . There are usually Hotlines to report elder abuse . I would start with the Federal Trade commission and see if social services can help you with a low income lawyer . Your sister has committed a felony .

As others others have suggested please file a report with Adult Protective Services and contact and Report to your local police department.
Assuming that your mother is over 65 please also contact the local Office on Aging and see if they have a listing of Elder Care attorneys who can assist at low-cost or perhaps even at least hear the story pro bono.
Wishing you and your family good luck and peace on this journey

HAve you called the police and reported the theft of your mother's asset?

HAve you called Adult Protective Services and reported financial abuse of an elder?

DI'd youask mom if she signed a Power of Attorney for your sister? If mom didn't, then you also report fraud.

Are you going to contact an attorney?

"My sister should be held responsible for stealing all of her assets. She knows I can't afford a lawyer. Momma doesn't deserve this."

What difference does it make to Momma, since she is living at your house with a 24/7/365 caregiver (you)?

I hope she is paying you what she can from her SS check. Is she? What was going to happen to the proceeds from the sale of the house -- were you going to get it, or was it to be split amongst you and your siblings? (If so, why? YOU are the one who is taking care of her.)

If your mother ever applies for Medicaid (someday her care could easily become too much for you), if it's within 5 years this house sale could affect her eligibility.

Let me see if I understand you correctly. Your mother has been living with you for six years. Was her house just left empty all this time? With all respect to you, you're leaving something out. No one moves an elderly parent or anyone for that matter in without knowledge of their finances or some kind of legal documentation (POA, conservatorship, guardianship, legal agreement) in place.
Your sister did not just get POA. That is not possible. If she was going that route, your mother would have had to appear in court or a lawyer either retained by the court or by your mother would have had to appear on her behalf. Your mother lives in your house for six years. In that time has she ever heard from a lawyer appointed to her because your sister is petitioning for POA? No one just gets awarded POA by the court because they want it.
This being said, your sister already had your mom's POA. Your mother made the documents in lawyer's office naming your sister her POA. She may not want to admit to it because she lives off of you in your house, but I guarantee if you get a look at those papers, your mom's signature will be on them.
You can put a timeline as to when she started acting as POA. Who was paying the bills at your mother's house for the last six years? The utilities, the homeowner's insurance, the property taxes, the mortgage if there is one?
Where does your mother's mail get delivered? There will be photocopies of cancelled checks included in her monthly bank statements. Somebody pays these bills so there's a signature on the written checks.
You have access to your mom's SS. How are you able to access this money if your sister is the one in charge of all the finances through her POA?
Ask for some bank statements and see if sister's been paying these bills.
Also, when someone has POA, they cannot just abscond with the person's money and liquidate their assets to spend any way they wish.
Follow the money. Start with the bank your mother's SS gets deposited in. Deal with them. They will have a copy of the POA on file otherwise your mother would be the only one able to withdraw from her accounts.
Mom did a POA. You could also be named on that document as well.
If you're not pack her stuff and drop her off on your sister's doorstep.

You can't "file for POA". It's something that your mom had to give, willingly.

Have you asked sis for an accounting of the money from the sale?

I have used the National Elder Fraud hotline at 833–FRAUD–11 
(or 833–372–8311) when my sister was being scammed. The people answering can provide local & federal resources to access and also offer to open a case number for further assistance.