30 August 2021
As it's more of a criminal case and most importantly the non-bailable one, you should lodge an FIR under section 380, 445 & 452 of Indian Penal Code, 1860 respectively against your relatives. And simultaneously file a civil suit for the possession of property u/s 6 of Specific Relief Act, 1963 and for the partition of your undivided property u/s 6 of Hindu Succession Act, 1965.
Section 380 deals with the punishment for a person who commits theft in any building, tent or vessel used as a human dwelling, or used for the custody of property.
Section 445 deals with when a person effects his entrance into the house by opening any lock in order to the committing of the house-trespass, or in order to the quitting of the house after a house-trespass.
While section 452 applies in case of house-trespass after preparation for hurt, assault or wrongful restraint. Though they only changed the lock but it clearly shows they're refraining you from entering into your property hence it amounts to house-trespass after preparation for wrongful restraint.
And as for the improvement or some modifications they've done on your property without your consent, they are not entitled to get reimbursement for that.