As far as outfits 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.
I would suggest avoiding big patterns and logos/brand names – when looking at the photos, the viewer’s eyes will be drawn to them.
At the end of the day, you still want to feel like yourself.
Here are a some portraits to see how different colours and styles work together. You might be drawn to a certain look and hopefully, you will have some inspiration from them.
If these are the look you like, here are a few Pinterest boards to view: https://www.pinterest.com.au/tara_leephotography/what-to-wear-for-families/