Taking a vacation with your own family can sometimes be stressful but if you are going on one that is going to combine families together this can not only be stressful it can be a little bit tricky to plan, far more so than if it were just you and your own immediate family. When you go with large groups within your family unit you are going to have all different ages and all different interests within the entire group so it will make it challenging.
When you start planning for something of this nature make sure that you ask all of the family members that are considering coming that they give you their ideas for the vacation well in advance. You want to make sure that each person feels they are involved in the planning and that their input is appreciated. You should try to make it so that the vacation plans allows for each person to have at least one thing they want to do included within the entire plan. However; still try to make most of the activities and things to do and see as enjoyable to as many people in the group that you can.
It is doubtful that you can take just one car so it's important that you ask someone else to be willing to drive as well so that you have at least two vehicles to accommodate everyone. This probably means renting if you don't have mini vans for the trip. But with a large group, everyone can pitch in when it comes to the finances. Having more than one vehicle will also make it easier to take one group somewhere while another group goes somewhere else when they are wanting to go see and do different things.
It's a good idea for you to try and rent a vacation home instead of motel or hotel rooms. In the long run this is going to be far cheaper. Make sure that you find a place that does not change it's price for the different amounts of people that you have. You might even consider camping if everyone is up to that. However; you may have older members that camping may be a little too hard on so that could be out of the question.
Make sure that some of the places that you are going to are very expensive, the people you are taking are going to appreciate the place and are going to enjoy it. For instance if your plans include a trip to an amusement park, don't take someone on that particular activity if they simply are not going to enjoy themselves, it will just be a waste of their time and money.
Make sure that you set up a buddy program. You should have every member of the group have a partner that they in charge of looking after and to make sure that no one gets lost or accidentally left behind when going somewhere. This is especially true for children of all ages.