Hi Madhu ji, let me help you real quick here.
The 6-acre property in question comprised 3 acres purchased by your father before 1955 and 3 acres inherited from your grandfather. Upon partition in 2004 through a registered partition deed, 5 acres were transferred to you, and your father retained 1 acre with a provision that, upon his demise, it would pass to the grandchildren. You subsequently sold 4 acres.
As my fellow professionals opine that the property inherited by your father is not ancestral but his self-acquired property. Consequently, he had absolute ownership and could partition it as he deemed fit. The registered partition deed executed exclusively between you and your father is legally valid and cannot be challenged at this stage. Your siblings have no rightful claim over the property. However, any challenge to the partition may be subject to judicial scrutiny.
Regarding the 1-acre land designated for the grandchildren, its distribution will be governed by the specific terms of the partition deed. If the deed lacks clarity, legal interpretation may be necessary.
For further legal consultation, contact me at adv.vishesh@icloud.com.