Funny thing is, if they weren't biologically cousins, the sister tag would still absolutely apply. Kirito was adopted by Suguha's parents, making them step-siblings, and they didn't even find out they weren't really biological siblings for years. In other words, in the form of tags, it's sister + incest + inseki.
Then you throw already biological cousins before that into the mix and... how does cousin + incest + NOT!inseki mix with sister + incest + inseki while still keeping the relationships clear?