My Family Wants To Take My 10 Year Old Brother To Another Country To Meet My Pdfl Uncle. Need Advice On How To Proceed

Hi Reddit. First time poster. Details will be changed/omitted for safety.

Context: My uncle has been convicted of assault against a minor 3 times throughout my life. I was a teenager when he assaulted my friend during a sleepover. He has been convicted and is on our local registry. He has since left the country and my family knows where he is, despite being wanted in our state. My family refuses to talk about it, and a lot of my family members do not know what he did, and have been told false reasons why he is no longer around. My father has been married 3 times and has had 2 kids, myself (30f) and my younger brother (10). My brother and his mom, Sarah (not her real name) live in another state and are financially supported by my father. He has lost custody of me and lost his home when my uncle Matt (not his real name) was convicted the first time.

The issue: I was mistakenly made aware that John will be getting married in the country he is living in, when Sarah sent a message to my family group chat specifiying that she had to be available to take her and my brother to the wedding in January. I was not aware of any wedding so I replied in the group chat, "what wedding?". I messaged Sarah, and my other uncle Greg (not his real name), privately to ask what wedding they were talking about. Both of them said the same thing, "Matt's wedding". I immediately sent everything I had access to, to Sarah so she can make an informed decision on whether to take my brother to this wedding. Her response indicated that she did not know about Matt's convictions, and wants to talk on the phone later to get more details. My uncle Greg started begging me to not make this a big deal. He said a lot but when it comes to the safety of my brother, and my young cousins who I have confirmed are also goin, I will do anything to keep them away from this monster. I really don't want to be the reason my family loses kids and faces legal charges, but this has crossed a line.

My question: I think I have enough to get my uncle extradited from the country he is in, but at the very least I don't want my family to allow the children to be around this man. Before I go nuclear, what legal recourse can I take if they take the kids to this wedding? Any advice would be greatly appreciated.

Location: California, USA