I am very sorry to learn of your mother's passing.
I am not sure precisely what these other attorneys told you. I will tell you that a hospital is liable for the actions of their employees, so if you can prove that they did in fact take or misplace your mother's belongings, then they are responsible to compensate you (her estate to be more precise) for the loss.
I can also tell you that this kind of claim has some fairly significant practical difficulties if you wish to pursue it. (1) you must hire a lawyer, you cannot prosecute a claim on behalf of your mother's estate "pro per" (without a lawyer). (2) you must prove the precise items or belongings that your mother had with her at the time that she went to the hospital (transfers to the ER, especially by ambulance, are often busy and confusing times, you must be able to provide admissible evidence to support the precise items that were lost, and the value of each). (3) you must also provide admissible evidence that the hospital (through their staff or employees) is responsible for the loss - you do not have to meet the criminal "beyond a reasonable doubt" standard, but you still must prove to a "preponderance of the evidence" that the hospital lost or stole your mother's belongings.
All of this is to say that while the hospital is responsible for items stolen by the hospital are the hospital's liability. However, you must prove each element to the claim - and as your mother is deceased and you are suing on behalf of her estate, you must hire a lawyer. This makes things more difficult.
If you have not done so already, do file a formal complaint with the hospital - most hospitals do work very proactively to deal with these matters (it is poor customer service to permit it to continue, healthcare works on a basis of trust between patients and providers and if there is a history of theft or other misconduct it destroys the hospital's reputation and efficacy).