It depends on the couple.
I got certified about 3 months before my (now ex) wife. When she took the class, she wanted me to go through it with her. I talked to the instructor and he said OK, as long as I didn't try teaching her anything ... this was his polite way of telling me to pretend she was just another dive buddy. I had to remember that advise several times during the class, as it's natural for a relationship to get in the way and for spouses to want to "help" each other learn things. But it worked ... mainly because we talked about it openly before the class started.
In my experience, many times when family members are in an OW class together they WANT to experience the class as dive buddies. Unless there is a compelling reason to break them up, I would be inclined to let them be buddies. However, if I see one of them becoming reliant on the other in any way, I will separate them and explain that I feel they will both get more out of the class by not doing their CW or OW coursework as a buddy team. This has only happened twice, and both times I found that honest, open communication ... emphasizing the safety aspects of self-reliance ... was the best approach to take.
At any level, when I have a buddy team who are family members ... especially if one is more experienced/competent than the other ... I always precede the class by letting them know that I'm happy to have them pair up as long as they let me do the teaching.
... Bob (Grateful Diver)