First, we are sorry for your family's loss. Losing someone while a loved one is incarcerated and unable to be there is one of the harder aspects of incarceration for everyone involved.
The answer is that it is possible but not guaranteed, and the decision rests entirely with the warden.
The CDCR has a formal policy for what is called a Temporary Community Release, which covers situations where an inmate needs to leave the facility briefly for a compelling personal reason including the death of an immediate family member. An uncle qualifies as immediate family under CDCR's definition in Section 3000 of the California Code of Regulations.
Your nephew needs to move quickly given funeral timing. He should request an urgent meeting with his counselor today and formally request a family death furlough. The counselor initiates the process and it works its way to the warden for final approval. The warden has sole discretion on whether to grant it.
Several factors affect the outcome. Your nephew's custody level is the most significant. Minimum custody inmates have the most straightforward path to approval. Higher custody levels require custodial escort, meaning a corrections officer accompanies the inmate during the furlough, and approval depends on whether qualified escort staff are available. If escort personnel are unavailable, the request can be denied on that basis alone regardless of the circumstances.
Inmates in administrative segregation, those with felony detainers, organized crime designations, high notoriety convictions, or those with seven or more years remaining to parole face additional restrictions that can limit the furlough to a funeral visit only under custodial supervision.
Time is the critical factor here. Get your nephew to his counselor immediately and make the request as urgent as the circumstances require.