doesn´t this apply for the "incest" tag?

Yes, seems that way...

They're just cousins, and cousins are accepted to marry by Japanese law.

So it's legalized incest. It's still incest.

Kitajima_Kaede and Kitajima_Sara from the Sono Hanabira series are (virtually) always marked with the "incest" tag, and *they* are cosuins. This is why the "cousins" even exists and why it is considered an appropriate tag to use with conjunction with the the "incest" tag...thus tags fixed.