Capital gain investment

I am assuming the father is not alive. The bonds can be purchased in your mothers name to save LTCG. A flat can also be purchased to save LTCG. An assesee can avail exemptions u/s 54 and 54EC both at the same time to save LTCG. That is, some invested in bonds and some in flat. 

Flat purchased has to be in the name of the mother. The 3rd brother can be named as second holder. If it is a flat, there are two options viz, joint ownership or associate membership. In both the cases, in the event of the main owner being deceased, the society will demand a succession certificate or letter of administration from the nominee or legal heir, to determine who will become the rightful and lawful owner of the said flat. Without these formaility, the 3rd brother cannot solely claim his rights over the flat. The society will not make him the owner of the flat without the consent of the other two brothers. The other two brother's consent or arrangement or settlement is required to settle the estate of the deceased. 

Dear Mr Mihir :

Thank you for your response. Yes my father passed away and subsequently the flat ownership came to my mother.

The 3rd brother intends to buy a plot (site) as a property and not a flat. 

So if we buy and register the plot in joint name with my mothers name first and joint name of the 3rd brother, then in case of her passing away, with the help of a registered document of consent / settlement / arrangement from the other 2 brothers, the property will be wholly in possession of the 3rd brother, right ?

the other 2 brothers have a son and daughter each. will they have any rights on this plot ?

Thank you again.

Property disputes can be very messy. I don't want to go into details here.

My advise is to ask your mother to prepare a Will stating all the details of her estate and in the event of her passing away, who shall be the rightful owners of her estate and at what ratio. Even if a plot is purchased, it should be in your mother's name, and she can prepare a Will stating the next owner of the plot. 

Prepare a Will, taking everybody in confidence and agreeing to the terms of the Will, so that in the future, there is no room of misunderstanding amongst the brothers. And everybody shall receive that is rightfully theirs.

Thank you again.

Would like to know that if this plot is purchased in the 3rd brothers name, with my mothers funds, how much tax needs to be paid ? i believe it is 20 or 20.6 % ?

The money received belongs to your mother. So if a plot is purchased in 3rd brother's name with your mother's money, your mother is required to gift the money to the 3rd brother first and then he can purchased the plot in his name. And to gift such a big amount will require a gift deed.

Since your mother will leave a portion of LTCG, she will be liable to pay tax of 20.6%.