If my mother has all her children in her will can the person that has leaved with her for the last 17 years contest it?

My mother wants to sign her house over to me because she is afraid that the person that has leaved with her rent free for 17 years will try to take it. My mother let her move in with her after my dad died because the lady did not have any where to go and my mother was lonely. Would signing it over to her children be require even if she had a will or could the lady contest it since she has been leaving with her for all these years?