What you wear to your session will dictate the whole feel of them. The simpler the better. Try to keep the number of different colours down to three or four. Colours should be similar, but not exactly matching on every family member.
Similar or complementary colour tones and fabrics work well. I find that pastels, whites and neutral colours work really well together. If you have time and the inclination, lay all the clothes out on a bed, and see if anything stands out as not fitting in.
Always stay away from logos/brand names and all different patterns. I am always here to help 🙂