I think this kind of sums up what your HD may be thinking. I don't know the answer, but I do know that on the few occasions my DH say no, I generally try to acquiesce.
You said it!!
When my DGD was 4 the family planned a trip to WDW. My DD, Kady's Mom, only had one request, she wanted to stay at AKL. Done. I was my DGD caregiver, so I was home with her and able to spend a lot of time researching what was our first trip back as a family since my own children were little. My first husbands sister wanted to come along. Fine. She loved Kady, wanted to experience her first trip, so why not? OMG!!!! Why??????? She was a frequent WDW visitor and had her own idea of how to vacation there. It was not what my DD and DSIL wanted, and it did not consider that my DH had not been on a vacation in 11 years. When you begin a trip with obvious differences, it can only go two ways.....better or down the drain. It did not get better. The resort? no. The
DDP? no. CL Level? no. Right down to how her friend used to book rooms was a discussion. Finally, before a full mutiny took place, I had to tell her enough!!!! We had already made decisions before she decided to join us, and she had a choice. Come with us, stay home, or stay with the niece and join us when she wanted. But she had to stop upsetting my daughter, or the decision would be taken out of her hands.
The trip itself was awesome! My DS and DDIL were able to join us as a surprise, and my first husbands niece and nephew did as well. DH paid for all OOP expenses for everyone, tips. meals, and alcohol. He never said a word while we were there until the last day. He said "Never again." Turns out my family all said the same thing. "If she goes, we stay home."
SO we plan a trip the following year, and do not include her. That was a nightmare in itself, and I ended up planning an additional trip so she and I, along with my niece could take Kady. That was a disaster.
OP- You have a choice in how you handle this situation, but beware that you do not fall into the trap I fell into. Your brother may say he wants you to plan, but what he is demonstrating is that he already has made his mind up about how you all will vacation. My sis IL said one thing, but her actions on both of those trips were in direct contrast with her words. It sounds like your brother is much the same way. He wants to be in control, and while that would be fine if both families wanted the same thing, your families are not on the same page. When you add that he is not Disney knowledgeable and wants you to be his guide, but has already set terms, all I see is a disaster in the making.
I think you have two choices. You can tell your brother what your plans are....and I see you are already making changes.... help him make his, and you meet when you can.
Or, and this is what I would do, incidentally if I could convince DH to go along, ...and I am a people pleaser...tell brother that if he comes he needs to understand that you are on two trips. Yours and his. You can meet when it is convenient, but you have made plans that are pretty much set in stone. I'll be honest, my husband would most likely choose to stay home unless I made huge changes in this trip.