This is a clear case of dowry death. The death has committed in the matrimonial home under unnatural circumstances. If the death has been caused within seven years of marriage in unnatural circumstances. Then it is certainly a dowry death.
The court will not grant bail if ingredients of dowry death exist
It is a settled law that when the essential elements of dowry death exist in the case the court will not grant the bail to the accused. In your case however, postmortem reports suggest that it is a suicidal hanging but the injury marks on the dead body proof that it is a postmortem hanging.
Section 304-B of the Indian penal Code defines the dowry death and provides punishment thereof. According to it the death should be committed in the matrimonial home in unnatural circumstances and within 7 years of marriage.
Ine Vishwajeet Halder Babu Haldar and others vs State of West Bengal  1 SCC 202; the supreme court explained the ingredients of the doll death as
- The death of a woman has caused by burn or fatal injury or otherwise than in the normal circumstances.
- Such death should have occurred within seven years of marriage.
- She must have been subjected to cruelty or harassment by her husband or any other relative of her husband.
- Such cruelty or harassment has caused in the connection with demand for dowry.
All these ingredients exist in your case therefore the court will not grant bail. The court generally does not grant bail in dowry death cases, if the death is unnatural and committed in the matrimonial home within seven years of marriage.
Demand of dowry
You have to prima facie prove that there was demand of dowry and the deceased had suffered cruelty in connection with dovery. As you have mentioned in the question, your son in law was demanding a car. This demand comes under the purview of dowry.
In Satvir Singh versus State of Punjab  6 SCC 633; the Supreme Court has held that the word dowry should be any property or valuables given or agreed to be given in connection with the marriage.
If you have agreed at the time of marriage that you will give a car. thereafter its demand from the husband of the deceased will constitute dowry. This evidence is enough to prove that there was cruelty in connection with demand for dowry at the time of death of the deceased.
If the accused are influential persons and they can manipulate the investigation, you can transfer the investigation to another district. You can file a petition before the High Court and the section 407 of the code of criminal procedure for transfer of case to another district. Fair investigation and fair trial is a fundamental right under Article 21 of the Constitution of India.