1. Ask Cougarboard for advice
2. Read all the snarky responses
3. Contemplate why you ever asked Cougarboard
4. Decide to hire a lawyer
5. Ask Cougarboard for advice on hiring a lawyer
But seriously, I would recommend at least talking to a lawyer BEFORE doing anything. CA landlord - tenant law can be tricky and tends to side with tenants.
You also want to be careful to avoid an adverse possession claim by your nephews so don't make any claim that they were living there without permission (at least until consulting with a lawyer first).