I would start out by picking some neutrals (either brown/tan based or black/gray based) and then adding 2 - 4 colors that you already have a lot of in your closets, that way you can combine new stuff with some things you already own. Have everyone wear at least 2 of the colors.
We like to keep things interesting so we went with 4 colors! We did brown/tan/ivory as our neutrals with mustard, teal, green and rust/orange. But if you like things to look more matchy, just go with two colors (in addition to your neutrals).
Layers and textures add visual interest. Think scarves, vests, cardigans, leg warmers, jackets, and hats. Lace, knit, or woven textures are subtle but beautiful.
Sometimes it's fun to have mom or a sister wear a bold pattern that contains several of your colors. To keep it visually pleasing, don't have more than one person wear a bold pattern.
And of course avoid words, logos, and other distracting elements on clothing.
Also, it can be a good idea to plan Mom's outfit first and then plan everyone else around that. That way you know you'll love it and feel beautiful and confident, rather than trying to throw something together at the end that goes with what everyone else is wearing.
