well no, recessive genes are a thing. which means that you typically need two copies of a bad gene for the genetic defect to manifest itself. usually you only get two copies of the same bad gene if your parents are siblings.

but inbreeding itself doesn't produce bad genes - it simply produces bad gene PAIRS. this is bad for the parents and the child in the short term but actually good for humanity in the long term because it speeds up the elimination of bad genes from the gene pool.

these bad genes can only be eliminated if their effects manifest themselves which will only happen when they form a pair so actually inbreeding is a good thing from that perspective.

also since you and your sister both carry two copies of genes not all your kids will end up sick. some of your kids will die and you will simply make extra ones and the ones that made it will have higher genetic quality than if you married normally and simply MASKED genetic defects rather than flushing them out.

of course i wouldn't risk it personally. that is i would rather have healthy kids of poor genetic quality than unhealthy kids of high genetic quality. but that's where surrogate mothers and so on can come in.

if you and your sister ( or daughter ) get along well there is no reason to not also be fucking ( except that of course you will be turned into pariahs )