Your son-in-law is the husband of your daughter. His son is your daughter's son and therefore your grandson, and you are his grandparent.
If your son-in-law's son is from a previous marriage, the son is not physically related to you. He is your daughter's step-son, he is your step-grandson. However, you may want to treat him like a grandson anyway, especially if he came into your life at an early age.
There is no specific title for that relationship. Their forenames or "my son's in-laws" would be appropriate.
Co-in-laws?
No.
Outlaws are wanted...... inlaws aren't.
Ask your spouse
petheriká
in-laws. My wife's mother is my mother in law. My wife's brother is my brother in law. (and so on)
No.
Inlaws.
Adams sons were called Abel and Cain.
but your brother is her husbande
Sounds like a wonderful Wedding Present for the new inlaws. The Phantom of the Opera in Toronto will be perfect and enjoyable.
Gloucester's sons are called Edgar and Edmund.
Yes, because he is married to Dorothea Hurley