So, the sister and her husband took advantage of mom's fragile mental condition and talked her into putting only her on the title of the house. Mom passed away shortly after.
On what date did your mother sign the deed?
On what date was the deed filed/recorded?
On what date did you learn this had happened?
What was your mother's date of death?
Please elaborate about your mother's "fragile mental condition," including the date(s) of any relevant medical diagnoses.
does she legally get the house and all inside?
Your mother gave the house to your sister, so yes, it's her unless someone can either convince her to do something else with it or proves in a court that something legally improper occurred. The contents of the house are a separate matter. If they belonged to your mother, then they are the property of her estate. If they belonged to someone else, then that person or those persons will continue to own them.
Are they guilty of elder abuse or conspiracy to commit fraud?
No one here could possibly render an intelligent opinion about this based on the limited information provided.
Should we go to probate court?
You've told us nothing about your mother's estate, so we have no way of knowing whether probate is or may be necessary. What is the approximate value of your mother's estate (not including the house)?
If you believe your sister did something wrong in connection with the transfer of title of the house, then yes, you ought to at least consult with a local attorney.