The 24-70 version I is particularly well known for having a curved depth of focus at the wide end. If this is the culprit lens, take a photo of something like a break in retaining wall ( 3-4 step stairway) where you can see both the horizontal focus plane - the top of a retaining wall, and the vertical - steps or vertical element of the retaining wall at f2.8 and notice the parts that are in focus as you move away from center of frame.

Once you see the curve, you will suggest that the group shots be done in an arc i.e. closer to the same distance from the film plane - sounds minor but in a large group shot the bride is 10 feet and the outsiders are 15-20, that is a lot of DOF for F4.