do sons have ownership, property in the name of mother?

Under Hindu Succession Act Property of female Hindu is her absolute property. In your case it may be possible that they may take a stand that, it is treated as Hindu Joint Family property, created out of joint contributions of all. If there is any evidence as to contribution of her son then take NOC from that son that the contributions he made is out of love and affection and not as a Joint Family Property. 

Regarding Documents Required to purchase a property in bangalore depends upon each case to trace its title.

1. RTC of the land in which the site comes (From 1968-present day). This is needed to verify whether land is affected from any PTCL act provisions, or from any Land reforms provisions etc. If such documents does not disclose preliminary title then Index of lands and other documents are required. Hence i said it all depends upon each case.

2. The DC conversion certificate duly verified from the concerned office.

3. The layout approval from the planning authority.

4. Building Plan approval from the local authority.

5. The upto date tax paid reciept from the local municipal bodies. And also khatha extract.

6. Form 3/ Form 10/ Form 9 depending upon particular cases.

7. Encumbrance on the land which can be best advised after verifying records.

8. Affidavit from her sons stating the land and building is her self acquired property.

9. If the property comes beside railway line, or National Highways then NOC from Railways/ NHAI is must before building constructions.

10. The preliminary title to be verified because bangalore is witnessing so many land frauds. (To satisfy -land is not govt land) 

ETC

For Registration requirements of documents are simple but for legal verification of title it is laborious task depending on each case.

HINDU SECTION ACT SECTION 14. Property of a female Hindu to be her absolute property
(1) Any property possessed by a Female Hindu, whether acquired before or after the commencement of this Act, shall be held by her as full owner thereof and not as a limited owner.

Explanation: In this sub-section, "property" includes both movable and immovable property acquired by a female Hindu by inheritance or devise, or at a partition, or in lieu of maintenance or arrears of maintenance, or by gift from any person, whether a relative or not, before, at or after her marriage, or by her own skill or exertion, or by purchase or by prescripttion, or in any other manner whatsoever, and also any such property held by her as stridhana immediately before the commencement of this Act.

(2) Nothing contained in sub-section (1) shall apply to any property acquired by way of gift or under a will or any other instrument or under a decree or order of a civil court or under an award where the terms of the gift, will or other instrument or the decree, order or award prescribe a restricted estate in such property.