Hmmm.... a few thoughts: I wouldn't worry too much about the back/rim light if you're using a white background. Two large soft boxes should be plenty for the group; both at 45 will give you even, flat light however, which may not be ideal if the MiL is not a fan of the studio look. How about one 30 degrees camera left (or right) as key, and a fill about 10-15 degrees of axis on the opposite side 1/2 - 2/3 stop down. That should give you a bit of shadow and interest without being too dramatic or "shadowy". A lot of the lighting however will depend on how you plan to pose them. Remember, if you're going have them seated, DO NOT let them lean back. butts on the very forward edge of the cushions!