My mother was with me for the entire year (she was on a visit visa for 6 months and later an extension,no ssn etc.. )

In the mean while my sister sponsored her and her EAD has already come, her GC is still in process and due in approx 2 months.
Currently she is back in India.

Can I show her as a dependent ? If yes what formalities I need to do with regards to ssn or itin

Dependent must be a citizen or resident. She is nonresident.
Even when she was with you for more than 6 months, she was Dual Status. Then she left the U.S. and became nonresident.