If he insists on it being a surprise, get a wad of cash from your brother and:
I suggest you buy some cheap Disney trinkets from the dollar store or party store for travel distractions, buy a simple blank page book or 4 x 6 index cards and small 4x6 photo album with stickers so they can decorate an autograph book. Hats and wet bandanas will help keep sun off and the wet bandana will help keep cool or frog togs. Maybe tshirts for the whole group. Buy some cheap rain ponchos, you will need them, Florida is good at 10-30 minute rain showers everyday, sometimes longer. Some Disney gift cards for spending at the parks. This is for the kids aspect.
Come up with strategies for lines and keeping the kids distracted and happy. Make sure your brother is aware of the lines and crowds and he can get some things to help distract.
For the wife; maybe he should buy her a spa day for the stress he will be giving her. Have a book ready with important passages marked. Give her a copy of the plans, including meals, talk her through everything when she is receptive for it. Print up some passages from here explaining how important rope drop is and early morning touring.
Maybe buy some id tags to place on the kids shoe laces in case of separation.
Have table service meals booked, it is getting late to get reservations, set them up without talking to your brother so you can have some reservations. Table service will be important for air conditioned breaks with low stress. If you can get some character meals.
Be sure to get fast passes at the 60 day mark for Disney resorts or 30 day mark for others. Us liners are always happy to help.
As you said mid day breaks, careful planning of rides, ROPE DROP EXTREMELY important. For all make sure they have comfortable shoes, sneakers, I recommend avoiding flip flops.
Maybe place a garden grocer order so you will have snacks and drinks waiting for you.
If have to keep it a surprise the more you have set up should help.
Oh and be sure that your brother gives you your own Disney card for a gift for all the hard work and stress he is giving you.