11 YR old Girl: Don't be JEALOUS her mom is helping "sick" chld.

Help! 11 yr. old girl is upset her mother is traveling along w/her sister & their mother w/very sick terminal child to Disneyworld for X-Mas. This is sick child's only vacation/wish in yrs. It's all about her last "trip/wish/vacation." She does deserve it, for she has endured much pain and heartache due to her fatal disease. This sick child is living w/her Aunt and family along w/her mom. Her mom need's the help of her sister and their mother. The hotel room is going to be too small 4 all but 4 poeple. The sick child need's quiet time and can't be out very much at all due to her disease, so it limit's the fun. It would be no fun for the cousin, but she is upset, angry, sad, and jealous and making it KNOWN to her mother leaving her feeling guilty as to not taking her. The child has school, as this trip will take place in week 1 of Dec. It is obvious it is not a option to take her due to the demands needed of the sick child and her mother need's the help of her sister.
The father of the 11 yr. old is upset as well, feeling everyone should go! I've heard the story and it is BEST for the sick child & financial wise to just have the 4 people go and give this child a "dream" vacation at the very least.
How can it be resolved? THis child is upset, but more "acting" out we think just because. It is really worrying the mom and now the father.
This family need's some help with this, and I'd like to offer it to them if ANYONE out there will help answer what to do/say? If U R a medical professional/homemaker/grandmother...WHO… help me find answer's to help them "help" resolve this sad situation. It's not that they don't understand the child's upset, it's just they seem to NOT know how to handle it and being a good confidant...I don't have any answer's for them. I

IF YOU CAN OFFER ADVICE, PLEASE DO SO. I'LL PASS IT ON TO THE FAMILY AND HOPE SOMEONE CAN FIND THE RIGHT WORDS/SCENARIO TO PRESENT TO THIS CHILD FOR COMFORT, AND SHE IS I SUPPOSE TO YOUNG (MAYBE) TO UNDERSTAND THE SEVERITY OF THIS ISSUE...MUCH APPRECIATED...THANK YOU. FYI: ill child is 18 YRS./bed-ridden/O2dependant/extremely weak but this is her wish.