Rules of inbreeding?

[kc261]

Rule #1: breeding siblings is worse than child to parent

I've seen this before, but I'd like to know WHY? Or see a source.

I've tried to figure it out before, but it seems to me like on average it is the same. Parent/child breedings will always have exactly 50% of the same genetic material. Siblings/sibling breedings could, in theory, have anywhere between 0-100% of the same genetic material, but it should be 50% on average. So what makes it worse?