My grand father kept the ancestral house to himself and divided the land and shops he had inherited to all his sons. This division (including keeping the ancestral house to himself) is mentioned in this document
This portion of property that was retained by your grandfather in the unregistered partition becomes your grandfather's own property since he duly partitioned the rest of properties to his children by executing partition deed accordingly and the same was properly acted upon by all the share holders.
Now this own property of your father, in the eyes of law shall be intestate property left behind by your grandfather upon his death because the transfer effected through a notarised document is invalid.
3. The house that my grand father kept for himself through action 2. was transferred by him to my grand mother just before his death through a notorized deed.
The transfer of property effected vide a notarised document is invalid in the eyes of law, hence it can be considered as intestate property and eligible for partition among all the legal heirs of your grandfather.
4. My grand mother executed a will for bequeathing the same to one of her sons (not my father).
Your grandmother has no proper marketable title in the property hence bequest made through Will is null and void.
5. This son transferred the property to his younger brother (not my father).
The subsequent transfer by the son may also be considered as invalid owing to the title dispute.
Can I claim a portion to this ancestral house and / or the land on which it has been built?
You and your mother and other legal heirs of your deceased father can claim partition and separate possession of your share in the property but if your father is alive, only he can seek partition of his rightful and legitimate share in it.