If it's not illegal, the safest way to test the waters is to tell your therapist, since they wouldn't/couldn't disclose it. Or find a related support group for whatever you're ashamed of. You'd be surprised how granular such groups get.

The analogy of having a prominent nazi as a grandparent on its own wouldn't turn me against a person, though. But I'd tell that person to critically assess the way they were raised to excise any generational brain worms.