1 . Police is bound to take your complaint and register a FIR on this basis. No witness is required for that. Relative is a competent witness and police can't ignore his statement.
2. You can complaint with higher officials against local police. It should yield results.
3. If not then file a petition u/s 156(3) crpc whereby the magistrate will direct the police to register FIR. Alternatively you can file writ petition in high court against police inaction.