for starters, their feelings are hurt because you & their brother don't consider them "worthy" to be in your wedding party.
While this isn't your fault or your problem, you might want to understand this to help the overall situation.
So these girls, whose feelings are hurt about not being in the wedding party at their brother's wedding - which is a position of honor that they obviously hoped to have and are now disappointed about not having - have just been asked to be food servers - - which is a position that is no better than hired help!!
Usually, when you are trying to appease people with token jobs in the wedding because they aren't in the bridal party - - it is a guestbook attendant, greeter, usher, someone to hand out programs, basically something that makes them feel *special* and a part of the day of the couple being married - - not a server to the guests!
While I don't think you are acting overly rude - - you are dealing with people who have had their feelings hurt from the start and you are now asking them to work your wedding - thus making them feel insulted on top of slight -, by giving them a position that is lower than just being a guest. I understand why they are declining your 'request' to work your wedding.
My honest opinion is you should back off before there is irreparable damage to your relationship with your in-laws.
Maybe even sit them down and tell them you understand that you hurt their feelings by not having them in the wedding and you just wanted to keep the bridal party size down and that you hope they will not have hard feelings about it in the future. Tell them you are happy to be in-laws with them and you hope everyone can get over any hurt feelings.