You can't lock your family in the house and hope they never get exposed to ideas you do not like.
Your best course is to have a strong enough relationship with them so they know they can come to you and talk about what they are hearing/seeing.
Good points--thanks for the replies!!
I just have this reaction to people I think are nuts....
He is a mediocre Den Leader, but he is taking the time to do it, so I give him credit for that.
His wife is a big lib as well, so she pushed him into the Den leader thing...they seem to want to overcompensate for their liberalism. I suspect he won't be doing it very long anyways.