If both parties are consenting adults, and appropriate measures are taken to avoid pregnancy, l don't see the harm. The only caveat would be relationships where one person holds some authority over the other, such as a parent/child relationship. In the case of siblings or cousins, I feel that they are on the same level and therefore their relationship would not be inappropriate.
While I have no interest in having sex with any of my own family members, I don't see the harm in other adults engaging in what is otherwise legal sexual activity. I do find the idea of parents having sex with their adult children a bit borderline, as they have a certain level of power and domination over them, but cousins and siblings generally do not.
For this reason I don't see a problem with, for example, a brother and sister (or brother/brother, sister/sister) having sex for the physical enjoyment of it and expression of intimacy. We hug our siblings and kiss each other on the cheek all the time and no-one cares. Why shouldn't adult-age siblings who are very close be able to take that one (large) step further and express their sibling love for each other with sexual intimacy.
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