If not using flash, -
Aperture Priority mode and
ISO 400 to 800
1. Using AP mode with the 55 - 250mm set on 90mm and f5.6, walk around with a big smile and cheerful approach, before the formal shots, and take plenty of candid close crop portraits. This will break the ice and get your subjects used to having you around them with the camera. You may be surprised at some of the nice shots you get.
2. For the more formal portraits try to use diffused
light (maybe through a nearby window) and if possible use a reflector for fill
light. A large white cardboard held by an assistant would help.
I would use the 18-55 set at 55mm and the smallest
aperture possible ie: f 3.5.
3. When taking the formal large group shots use the 35mm
lens set on f8 and set at the hyperfocal distance of 8 meters. (or focus on someone in the center of the group). Use a
tripod and the 2 second
shutter delay to avoid camera shake. You must tell the group that you are going to take about 5 shots and that they need to be as still as possible during these shots. The reason for this is that in PS Elements 9 or later you can go into the "Guided Processing menu" and choose "Group Shots" and this will allow you to choose a principal shot and them move heads from the other group shots to remove closed eyes, frowns and other blemishes.
If using the flash I would use
ISO 400 and f8 at 1/200sec in manual mode and the flash set on TTL mode.
Try some test shots with your family, you will get comfortable with the setup and end up with some nice happy snaps in the process.
Good luck for the shoot.