Going with ones "guts" is what children do. We are supposed to use our logical mind as adults. I am still working on this discipline. Your sister might try to also.

In Sunday School as children they are taught the saying, "Jesus said it, I believe it, that settles it" -- or the variant, "The Bible says it, I believe it, that settles it". This allows them to un-ironically elevate dogma above facts and reason. And since they believe that god uses their "gut" to communicate his will to them on matters that are not specifically addressed in scripture, that's not much of a stretch either. Especially when "mere" human wisdom is actively decried by the Holy Book. "Trust in the Lord and lean not unto your own understanding", etc.

Life is too short to spend it constantly "grated on" by such things. Just let them be as they are and move on.