Most large families split into informal social groups based on birth order, gender, or both. These can be highly transitory during childhood and adolescence, but tend to formalize towards adulthood. Adeline is the 6th child, right at the center of the brood. I think she always considered herself to be one of the "grown" children, and when her elder siblings began to marry, she developed fast friendships with their wives. It makes a lot of sense that she'd see these women as her friends, and perhaps feel a closer connection to their children. especially since she helped raise them.
I think she just doesn't have the sense of seeing the wives of her younger brothers as contemporaries. They are all women whom she knew as very small children, and she's been transparent about how she continues to see the Campana sisters in particular as the little girls that they were for so many years. However, newlywed Addie at 29 is now far closer to the life experience of Campbell (aged 23, married two years with no kids) than she is to Rachel (aged 31, married for 13 years with 7 living children). Perhaps things would have shifted if she'd remained on the Morton compound, but by moving off-site, she's unlikely to change her sense of allegiance to that older generation of SIL's. The only thing that I could see changing this is if she and Michael move back in with her family, or if she and Campbell get pregnant at the same time. That kind of unique shared experience would certainly encourage a closer relationship.