I was happy to see you plan to take family pics including the in-laws as well My husb is from a family of 13 children. When there's a get-together, they take brother and sister pic's.(parents are deceased) I'm not the only "in-law" that feels slighted. The spouses are 1/2 of the team creating and raising the next generation. The sister-in-laws (9 of us to begin with) are the ones to plan, cook the food, buy the gifts, and in some cases see that our husbands attend. Some of us attend even if our spouse (no excuse) doesn't. I feel we've become an important part of the family as well.
GOOD LUCK with the party. I'm sure your FIL will appreciate your efforts.