There is no criminal case regarding the death, so there are no grounds to refuse to release the body to relatives.
The human rights project OVD-Info has launched a petition collection service: its dedicated service allows you to submit documents to government agencies in just a couple of minutes.
In less than a day, more than 12 thousand requests were sent to the head of the Investigative Committee.
They indicate that Navalny's relatives have still not received "any documents confirming that an investigation or criminal investigation has been conducted into the death."
This means that the investigation has no legal grounds to refuse to release the body to relatives.
You can sign the appeal here.