Your question makes little sense. Help mom find an attorney.
Anyhow, to establish ownership, you have to have a chain of title. There are companies, not coincidentally called title search companies, that will do that research and even sell you insurance warrantying their findings. This insurance is called, wait for it, title insurance. I wouldn't even take property from my sainted aunt without title insurance.
Anyway, I suspect the ownership interest your grandmother had isn't at issue (though the title search will verify that). The issue is how the property was conveyed from grandma to your mother. Grandma was free to do so without regard to the relationship of the recipient, other possible "future heirs", or whether the recipient or others contesting it had "done anything" for the grantor.
The issue is:
1. Did grandma legally convey the property by deed to your mother.
2. Did she do so through some fraud or duress directed at grandma, and grandma was competent to do so at the time
If the property was legally deeded and there was no fraud, duress, or incompetence involved, sister isn't going anywhere with her suit.