Have traveled with all combinations of friends and family...
If you can avoid the extended family trip I would suggest that first because:
(1) Your first trip with your GDD, I would not want to share that experience. With the other kids it is likely some of those special moments may even be lost.
(2) Is your son/daughter going or just grand? If they are, maybe they want this trip to be just your family?
(3) The one thing you will be guaranteed is the trip will not be like your normal trips and bound to bring some levels of stress, possibly hard feelings.
If me I would be telling her that
"this is a special trip since it's our first with our GDD and I don't want to share those special moments, maybe next time."
We've had many learning moments with extended family and I suggest:
(1) Stay in different hotels, this gives your down time more privacy and away time. This gives your GDD time away from the other kids.
(2) Staying in different hotels means meeting at parks, not bus stops. You will gain a huge amount of park time because if they are late you just keep enjoying yourself and they will catch up with you. If in same hotel, set meet times at parks, never anywhere in hotel or at bus.
(3) Book your food reservations with just you all for most the time and coordinate a couple meals together (at table where reservations are needed).
(4) Be clear on the parks you select for which days, if they want to go they can, if they don't - make is clear that it is fine to split up some days.
(5) Do not change your choice of tickets for them, say you do Hoppers and they don't want to ... fine, you spend half a day together.
(6) Do not agree to supervise or take her Gkids to parks should she become tired or not interested later. Tell her she should do a pool day with them. This is your time with your GDD.
(7) Def plan time with just your family whether it be park day, special meals etc. and clearly let your sister know that she should make her own plans during that time.
Good Luck
