In Europe, your usual choices when you disagree with the dominant church are to get over it, become atheist, or - more rarely - change faith altogether (as in to Islam, Buddhism, etc.) because, more often than not, you exhaust Christian picks after going through Catholicism and the local Protestant flavor or two.
In contrast, the US offers a very wide variety of Christian flavors (and Christian-based sects). So when, like your sister and her husband, you disagree you can just pick another church flavor down the block that is mostly the same with a few twists. You're still nominally Christian though.
The difference between the two is sometimes put forward to explain the contrast between the high number of atheists in Europe and the high number of religious people in the US.
If a person truly believes his religion, it has major implications for his/her notion of what it means for humans to thrive, what activities are moral/ethical, and the goals of human society.
It would be very strange indeed if those beliefs had no bearing on one's politics.
I don't know. I see good arguments for and against.