1. Can I get back my land and how?
First you talk to your cousin about this and ask her to at lest give back your money that have invested in the property with interest from that date or a portion of property registered on your name.
If she is not agreeing to oral and peaceful talks, you may plan to issue a legal notice demanding your property based on the documentary records in your name.
2. Can I file FIR against them in police station?
The police may not entertain any application or complaint in this regard because it is not only a civil case but also very badly barred by limitations for filing a criminal complaint.
3. Can I get any help from panchayat?
What is panchayat doing here?, They are not any law enforcing authority on all such issues.
4. Do I have to file a fraud case against them?
Not maintainable.
In my opinion, even the civil legal case may not be maintainable because she use the weapon i.e. "law of adverse possession"