Do you have any sort of source on that? Or personal experience that leads you to believe this?
I'm curious because I've seen people say this, and I've seen people say the exact opposite. I tried to figure it out, and I think mathematically it is equal. Offspring & parent share 50% of their genes. Sibling & sibling share 50% of their genes (at least on average). So I don't see why one form of inbreeding would be better or worse than the other (again on average).