1. The property was a leased property and your mother was a leasee and not absolute owner of the land,
2. So, her will, if any, in connection with the said nlease hold property is invalid,
3. Since probate is not mandatory at Delhi, you can not raise this question,
4. You became the joint owner of the property alongwith your other siblings only after the property was made free hold and DDA registered deed of conveyance was issued in the name of all the co-owners,
5. You are now the joint owner of the said property and your mother's will is invalid for reason stated above.