Common misconception, it is higher on a relative risk basis when compared to non related parents, but on the absolute risk sense it's still a very small chance of issues at least for 1st generation inbreeding, the risk rises considerably if compounded.
At the other end of the scale there are risks associated with outbreeding, inter-racial couples have far higher risks of birthing problems and health issues. and for subsequent generations the originally evolutionarily adapted gene complexes that balanced each other out are thrown into complete chaos.
A simple example to visualise is the genetics for tooth size vs jaw size; think what would happen with large teeth in a small jaw or small teeth in a large jaw.
Although my example is a bit of an oversimplification the principle can be applied to other organ systems such as the brain, these issues are quite real.
Another example could be genes that predispose to cancer in one population which over millenia genes that down regulate this risk have evolved within that population. There have even been a few studies that found more homogeneous populations actually had lower cancer risk (I speculate this might also relate to tissue type compatibilty).
3rd-4th cousins have even been shown to have higher fertility which might be an indicator of an optimal breeding range trading off the negative effects of both inbreeding and outbreeding depression.
To be clear I'm not actually advocating for breeding your sister but the risks of doing so are over blown as some form of scaremongering, people even freak out about cousins marrying which has even less risks involved and has been practiced by people in pretty much every human culture, the associated problems only come from compounding intergenerationally.
It's not 25%, if you work through the actual probabilities it's lower than that. it's only 25% chance that two sib;lings will share the same allele from one parent. if these siblings had a child you would only have a 6.25% chance of the child being homozygous for this allele and double if they share both parents. After that you still have to calculate the actual risk of having significantly deleterious recessive alleles in the first place.
Yet people with actual genetic diseases still go on to have children? The number of estrogenic, mutagenic and carcinogenic toxins in our environment continues to grow but people do little about it. The exposure level of microwaves from phones and wifi in a classroom is enough to permanently damage the egg follicle cells of a teenage girl, but the effects from that won't properly be detectable for maybe 20+ years?
" It depends on the degree of how related you are, siblings will have a bigger chance for the kid to come out normal than what people think (obviously the chance for deformities is twice as high than having a child between unrelated people) but you're usually safe, now if the children of said incest also did it than you're in for some The Hills Have Eyes type mutant people.
