They would be whatever they would be with your real siblings+in-law.
https://www.google.com/search?q=nephew+in+law&hl=en&sxsrf=ALiCzsbCVQak5qf0vIbEUtwRnOjg-VjFWA:1671139967946&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjV6aLByfz7AhXFFVkFHXxLDdAQ_AUoAXoECAEQAw&biw=1858&bih=1009&dpr=1#imgrc=4AfVpWuQ4lC9YM
So niece or nephew in law.
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