So bit of a backstory- since covid my sister's partner has fallen down the rabbit hole of conspiracy theories. It started with world economics forum conspiracies but is now full blown chem trails, lizard royal family, flat earth....you name it! Because of how aggressively he preaches at people he has isolated himself from all friends and most family members. Initially my sister was alarmed and concerned, but overtime there has been quite a bit of acceptance to these views. She basically thinks he has an 'amazing mind' and is right in a lot of his theories. I think there is quite a bit of manipulation going on. My sister and I maintain a good relationship by steering clear of certain topics. He has recently given up his job so he can focus full time on his 'research', so she is supporting them all (him plus their young daughter). There is talk of him starting his own business, but details are vague on what this would be. I have become recently more concerned as their daughter is due to start school next year. She thrives at preschool at the moment. The plan is for him to provide her education home schooling, along with running the 'business' on the side. I'm so worried that he will fill her head full of these extreme ideas....how can someone teach a child when they think the earth is flat?? I have broached my concerns with my sister, but she is having none of it...thinks the sun shines out of his backside basically. My understanding is that there is quite little monitoring from the local authority, and that they don't have to stick to the national curiculum. I'm at a loss on what I can do, without isolating my sister further from her family. Should I just butt out and support her and my niece however I can? I feel so sad, especially for my poor niece.