To repeat, although it is insanity to expect different results:
No, because there is no legal relationship and it doesn't specifically include such relationship, it isn't included in the exemption. You are correct, it isn't difficult, at least it shouldn't be.
You want legal advice? Here is a response from an attorney:
https://www.avvo.com/legal-answers/intrafamilial-transfer-of-firearm-in-california--4895872.html
The transfer of a firearm between a parent and child or a grandparent and grandchild is exempt from the dealer transfer requirement.
The exemption does not apply to step-children/step-parents, brothers, sisters, aunts, uncles, or cousins.
But wait, there's more, how about directly from the CA DOJ?
Public Where do I find laws regarding the possession of firearms? I'm not sure whether I have a California record that would prevent me from owning/possessing a firearm. Is there a way to find out before I attempt to purchase one? What is the process for purchasing a firearm in California? How...
oag.ca.gov
Can I give a firearm to my adult child? Can he/she give it back to me later?
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Yes, as long as the adult child receiving the firearm is not in a prohibited category, pdf and the firearm is legal to possess (e.g., not an assault weapon). The transfer of a firearm between a parent and child or a grandparent and grandchild is exempt from the dealer transfer requirement. The exemption does not apply to step-children/step-parents, brothers, sisters, aunts, uncles, or cousins.
If the firearm is a handgun, the recipient must obtain a Handgun Safety Certificate prior to taking possession and must also submit a Report of Operation of Law or Intra-Familial Handgun Transaction and $19 fee to the DOJ within 30 days after taking possession.
The same rules apply to the return of the firearm at a later date.
(Pen. Code, §§ 27870-27875, 30910-30915.)
So please tell me your basis for claiming that step anything is included in the exemption. A simple search would find what I presented. Asking the CA DOJ would find this result as well. Assumptions are generally a bad way to conduct business when legal aspects are involved.
I suspect that the attorney just copied the response from the CA DOJ, but since it is presented by an attorney, it is still legal advice