Cousin not allowing me use land came through partition

Dear Sir,

My cousin brothers not allowing to use our 15 agri land which my father (S5)received through family partition. Our family settlement had done in year 1974.

My grand father had got 6 (S1 to S6) sons and 3 daughters (D1,D2,D3)

- Grand father self acquired 60 acres agri land (GF-L) two houses (H1 and H2) in year 1953

- Eldest Son (S1)  had self acquired 30 (15+15) acres land (S1-L1 in year 1966 and S1-L2 in year 1970) and one house (S1-H)in year 1971

- S1 had dispute with S2 year 1965 and went out of joint family. Started his own business.

 Family settlement agreement done in year 1974 and details are:

1) S1 had given  his owned acquired land (15 acres land (S1-L1) and house (S1-H)

2) S2 had got portion (15 acres) of land and house (30% in H1) from Grand father self earned property

3) S3 had got portion (15 acres) of land and house (30% in H1) from Grand father self earned property

4) S4 had got portion (15 acres) of land and house ( half in H2) from Grand father self earned property

5) S5 (my father) had got portion (15 acres) of land from S1 (his eldest brother S1) self earned property and portion of house (30% in H1) from grand father self acquired property.

6) S6 had got portion (15 acres) of land and house ( half in H2) from Grand father self earned property

-No property had given to 3 daughters (D1,D2 and D3)

- Family settlement document had not registered and S2 had not signed this document.

 Grand father died in year 1982.S1 died in year 1998, S2 died in year 1988, S3 and S5 had died in year 2013.

S1 had 2 sons (S1-A,S1-B), both of them married. Recently got into family dispute since then S1-A and S1-B took our 15 acres land and asking me to go court to get this land.

My whole family runs on income coming from this 15 acres land. Can you please help and advise me on following points:

1) Does my cousin brothers has got rights to claim back their father earned property?

2) What is the best way to get back our land through court of law?

3) Both cousins brothers threatening other cousins to give portion of grand father property otherwise they will      go to court to get their portion of grand father property. Is it possible?