We're in the same boat... My sister is a single mom & so my parents watch my nephew a lot. She's an atheist. They're the speaking in languages type Christians.... And they try their absolute hardest to indoctrinate him. A couple weeks ago my mom tried to pass it off as them "exposing" him to other beliefs. He's FIVE. Five year olds don't have the ability to process that and decide for themselves. So.... They won't be watching mine other than a few hours here & there. And if they try to tell ours how to believe, I'll happily start telling my 16yo brother all about ours. Lol.
(Am slightly irritated with them right now... They borrowed our vacuum because theirs was broken. When my dad brought it back yesterday, he didn't bother even knocking and just walked right in. Getting ready to put a sign on our door that says "Knock. You don't live here.")