I think this kind of morality is ultimately subjective and it also heavily depends on direct family size, what family members you interact with, how often you see them, and you can't make a sweeping statement.
The "incest bad for genetic reasons" falls flat here given that your third cousin is more than adequately distanced from you genetically (second cousins are arguably so too, but it's more morally iffy since a lot of people know their 2nd cousins more than their 3rd and see them more as family), though obviously if your lineage keeps doing incest things aren't going to go well
