mashaAllah and mabrook.
True that it is allowed in Islam, and that is just what it is - allowed and nothing more, nor less.
However, same genetic defects in both families can result in a greater likelihood of these defects turning up in the offspring. This is a scientific fact, not a superstition.
These days, I am opposed to marriages between cousins because in most Muslim families, marriage within the same genetic pool has been the norm for centuries, and I have seen some evidence of problems turning up.
So I think there should be a little more tilt towards diversification of the genetic pool.
Ideally, a DNA test for known carriers of geentic defetcs should be carried out for prospective bride and groom, whether cousins or not.
May Allah bless this couple and families, and protect them and their offspring.