First and foremost, wear something you feel good in! The better you feel the better your portraits will be!
Second, matching prints or matching in general is not advised. Being all matchy-matchy looks dated really fast and takes away from the individuality of each group member.
Our goal is to bring out your individual personality as well as the personality of the family as a whole. It is tough to bring out your individual style when everyone is wearing the same thing! Try to coordinate rather than match.
Wear something comfortable that you can move in. Avoid loud patterns and prints or large logos, these sometimes look strange in pictures (especially stripes) and logos will make your pictures look dated quicker as brands and trends change quickly.