I only counted 6 people?

First, I would make the 15 year old carry all of his own gear and some of the group gear. And, depending on his size, maybe more. That should help you keep up with him. The 12 year old girl should be able to carry all of her own stuff. Most girls are almost full grown by 12. My 12 year old daughter had no problem with carrying her personal stuff when she was 11. Bare minimum should be sleeping bag, clothes, rain gear, water, and food. Make her brother carry her tent.

Second, bring lots of baby wipes. They tend to help girls (and boys) ease into coping with the wilderness. Also, you will probably have to teach your girls how to pee and pooh in the woods.

Third, bring 2 glow sticks, one for each night, for the 5 year old. Hang it in her tent. My kids seem to like a "light" on all night, especially in a scary wilderness setting, until they are about 7 or 8.

Go on day hikes before your trip. This will help gauge their hiking ability, and help you plan better.