They probably won't look any more similar to each other than you and your cousins though. There was this thing about sets of twins who did that. It's late and I don't want to find a source
If the twin girls had children their babies would be cousins to each other. 'Genetically' it's as if they are sibling because they would have very very similar genes but they are still cousins. They also probably wouldn't look that similar (the babies) to each other (or at least not any more than you and your cousins do) because of gene crossover and recombination
This is in no way a paradox. Also there's no guarantee the offspring will look alike, given that the offspring of one couple could inherit their looks from the father, while the offspring of the other could inherit the looks from the mother.
If you were to take their DNA and compare it, someone without advanced knowledge of their relationship would conclude that they were siblings.
But that doesn't mean that they would necessarily look like siblings in person. As identical twins age, they actually get less identical as they are both exposed to different things in their environment. And the type of identical twins they are can make a difference as well.
You could technically have one set of parents with all blond kids and the other could have all brunett. The physical charicterisics would be as diverse as regular cousins.
Edited to add:
Mary Kate and Ashley Olsen are fraternal twins.
This happened with my grandparents. My grandmother was a twin and I knew it all my life. I didn't know my grandfather was a twin until his funeral. My brothers wouldn't stop crying and hid.
But that doesn't mean that they would necessarily look like siblings in person. As identical twins age, they actually get less identical as they are both exposed to different things in their environment. And the type of identical twins they are can make a difference as well.
You could technically have one set of parents with all blond kids and the other could have all brunett. The physical charicterisics would be as diverse as regular cousins.
Edited to add:
Mary Kate and Ashley Olsen are fraternal twins.