This is my response to you:
1. First make sure the apartment you are purchasing is registered;
2. If it is not registered the subsequent sale will also be void;
3. Therefore register the apartment in her name and then later you purchase it;
4. Secondly, you must take NOC and an notarised affidavit from her stating that she does not intend to transfer the apartment to her children's names in the future and the children absolve the claim over the apartment (she being their legal parent and guardian can make such an affidavit);
5. Then you can also get an NOC from the society/administrator of the building;
6. You must also ensure that the land related papers are in order. Approach the sub-registrar or collector and retrieve the same from there.