How about this for the moms and sisters/inlaws with kids ... (and maybe alison) -
Black dress or black top with black skirt - knee length or mid-shin (I'm hoping that will make it look more summery) in a lighter weight material.
Add a colored cardigan (1/4 or 3/4 sleeve) - berry colored - red, purples, dark pinks.
And sandally shoes either black or the color of the sweater. (Heather, if you want, you can wear some of the crocs you bought. It looks like you got every color of the rainbow. if you dont think this is funny, see milehighmerrills.blogspot.com).
I'm hoping that will be easy for every one to be somewhat matching, and I'm hoping it will be easy on your time and wallets (maybe you already have all or part of this in your closet).
Let me know what you think ...