Details of property 100% ownership but in actual it was 50% what to do ?

Hello,

 My mother bought a shop couple of years back in 2011. Full payment was done by her, But in the agreement her daughters (my sisters name) is also there. The agreement does not mention the breakup of ownership...so by default it is assumed 50% each.

Now in all returns of my mother the ownership is shown 100% and the rental income also is shown in her accounts. My sister has never shown this property in her returns nor any rental income.

Now my mother wants to gift her portion 50% to me (son)..so what should be done when next year the returns are filed?

In my Moms return(ITR2) and my and my sister's return what should be shown under property section ?

Also when we (sister and me ) sell from when  will be capital gains be calculated ? from date of gifting?

Thanks