Hi
1. First of all you should I file an FIR for the lost the documents to comply wit the legal requirements and to avoid any future legal trouble in the next generation of your legal heirs.
2. Approach the registrar office where you have registered the WILL apply for a certified copy, then get the WILL executed by probating it .
3. The release deed from the legal heirs /siblings are the best option if your siblings are agreeable to it.
4. If you are getting a registered release deed to transfer the property, do also get the certified duplicate of the WILL from the registrar office.
5. As long as your siblings are agreeable to the transfer and they are alive it is fine, but you need to anticipate any future legal claims of the property of your mother by her grand children in the next generation which is also very unlikely.But it is advisable to be in a safer side and secure the property iwth all sufficient legal documents of clear title. .