Selling a Flat which is obtained by Gift Deed in India (Mumbai)

My sister and Brother-in-Law brought the flat in Mumbai on 17th Oct, 2009 in 15 lacs.

They gifted the property to my Mother on 5th April, 2013. My mother paid the Stamp Duty and Registration Fee.

Now, my mother wants to sell her property in 35 lacs.

I have few questions:

1. Is she eligible to sell the property?

2. If she sell the property now in 2014, will it be consider as LTCG or STCG?

3. What will be consider as Capital Gain?

a. Is it selling price (35 lacs) – cost price of my sister (15 lacs) = 20 lacs
b. Is it selling price (35 lacs) – cost price of my mother (0 lacs)  = 35 lacs

4. What will be the date of purchase be consider to calculate the Capital Gain?
a. Is it the date of purchase by my sister (17th Oct, 2009)?
b. Is it the date of purchase/gifted by my mother (5th April, 2013)?

e.g.
If she sells the property in 35 lacs, how much will be the tax.

a. Since it is not the Sale Deed, will the purchase date be considered as 17th Oct, 2009 and she has to pay tax of LTCG (around 20%) on 20 lacs profit? it means 2 lacs.

b. If not a, will the purchase date be considered as 5th April, 2013 and she has to pay tax of STCG (around 30%) on 20 lacs profit? it means 6 lacs.

c. If not a and b, will the purchase date be considered as 17th Oct, 2009 and she has to pay tax of LTCG (around 20%) on entire 35 lacs rupees (since the purchase price is zero, is the net profit be consider as entire selling price i.e. 35 lacs)? it means 7 lacs.

d. If not a,b and c, will the purchase date be considered as 5th April, 2013 and she has to pay tax of STCG (around 30%) on entire 35 lacs profit (since the purchase price is zero, is the net profit be consider as entire selling price i.e. 35 lacs)? it means 10.5 lacs.

Please let me know the rule and section in the law, which reflect this case of property transaction

Best Regards,

Ajay