How did your mother get the property in the first place? Did she purchase with her own earnings? Or did she get the property by gift deed or through inheritance?
First the cost of acquisition of the property has to be indexed and the gain over the indexed cost is deemed as the taxable capital gain. If the new property is purchased in joint names without indicating the share of each person, the first named person in the document can claim exemption from capital gain. If the share of each person is indicated, then each of the joint holder can claim to the extent of his/her respective share. In the case of joint holding without indicating the share of each person, it can be claimed that the name/s of person/s other than the first named have been entered for such purposes as inheritance, managing the property etc.
If you and your mother jointly purchase a property indicating the contribution of each of you, your mother can claim exemption to the extent of her own share. For instance if her share in the new property is equal to or more than her indexed cost of acquisition of the property disposed of, then she can claim full exemption.
If your mother sells the property and gives you all the cash, she would be liable for full capital gain tax, as though she has not purchased any property.