You seem to believe that your wife’s ex wants to be in these photos partly as a way to “control her life,” and partly as a way to create a tableau of the three of them as “one big happy family.” But let’s consider some other possible explanations.
Start with the fundamental fact that divorce is a decision made by adults that creates significant changes in their children’s lives. To name a few, there’s often a new home to deal with, less time spent with each parent, and an adjustment to new romantic partners or step-parents (whom the kids may or may not like). But the one thing that doesn’t change—the one constant in all this—is who the children’s parents are. The parents divorced each other, but they didn’t divorce their children.
In other words, this 6-year-old’s parents are still her parents, regardless of whether they’re married to each other. This means that for her, these pictures—at birthdays, a first day of school, graduations, and other occasions—aren’t “not-a-family photos” or photos of people pretending to be “one big happy family.” They’re simply pictures of her and her parents celebrating something joyful together.
Her father likely feels strongly that there’s a benefit to having both parents in these photos. He may think of how uncomfortable it might be for his daughter to have to take two sets of photos every time there’s a celebration—one with Mom, one with Dad—and how she might wish that despite the divorce, she could just be “normal” and take a picture by the cake or in front of the school with both parents at once. Her father may believe that seeing her parents work as a team on these occasions makes her feel loved and safe. He may imagine her as an adult, looking back at photos and remembering how hard it was to worry about the adults’ feelings on these otherwise happy occasions—having to deal with the politics of who gets to be in which photos—when all she wanted was for her parents to get in a picture together because they knew that the day wasn’t about them; it was about her.
I can’t tell you how many children of divorce later come to therapy and describe the sadness they felt when their parents couldn’t manage this on their behalf. Each photo of an important milestone became a reminder of the pain of the divorce—the loss was right there in the negative space where the other parent should be. They felt as if they were living two separate lives, and in each of these lives, the other parent had been completely erased.
Not all children will feel this way, of course. But enough do that you may want to consider this when thinking about why her father insists on this practice.
As for whether he’s trying to control your fiancée, I don’t know what he did or didn’t do in their marriage, but I can tell you this: In couples therapy, whenever one person is labeled something by the other—controlling, passive, angry—often it turns out that both partners share these traits, even if the presentation is different.