1. Since the property is the mother's self acquired property, she has absolute right to decide about the property to either retain or to sell, gift, etc.,. In the instant case, she wants to sell the property to you. During her lifetime, she can sell her self acquired property to anyone, including you and there's no need to get permission from anyone, including her children, her daughter-in-law, son-in-law, grandchildren.
2. Anyone can raise voice against her selling her self acquired property, even though apart from her, no one has any right over her self acquired property.
3. Those alive include kids of one deceased daughter, daughter (alive), daughter-in-law and son's daughter and their signature is not required, as the property you are buying is the self acquired property of the seller.