They are not siblings in any sense of the word.

There is a lot of stigma about dating family members for a good reason. There are real, horribly negative biological consequences if you have a child with a member of your immediate family, and the stigma is probably a good thing for the continuation of our existence as a species. While almost everyone believes that biological siblings should not date, everyone has a line for what they think is unacceptable and not. If you found out that you and your partner of five years were third cousins, would that necessitate you break up? What if a couple who have been together for 10 years found out they were half-siblings? If someone dates a cousin they are not biologically related to (for step-grandparent reasons) but grew up with, would that be worse than dating a biological cousin they didn’t grow up with? These are fun questions to ponder hypothetically, but when they’re your real life, it gets a lot less fun.