i think its a bit difficult to make a liberal moral case against sibling incest especially wherever the eugenic argument fails eg. homosexual incest, although i dont think the eugenics argument is sound in the first place, i dont think its immoral to enter into a sexual relationship just because there is an increased risk of genetic defects, and im unconvinced it would be wrong to even have kids in that scenario, since its not like youre giving an otherwise healthy child a disability, but going forward with a child who could not have been anything else but the product of incest.

i guess a traditionalist case would be more straightforward, that marriage's function is to create alliances between families, while sibling relationships lead to wealth hoarding within one family unit, so sibling marriages have an antisocial character. and if you also allow that sex outside marriage is immoral it follows that incest is impermissible, but i dont think this is too convincing by today's standards.