Matchy-matchy

I remember my mom & Gram sewing matching outfits for my sisters & I to wear. I am the oldest, and there are 18 months between me & my next sister, then there are 2+ years between her & the next sister. I mean, I don't specifically remember anything say, before I turned 4 or so, but around 6-7, I do, and I began to not be impressed. I didn't want to match babies. I didn't like the 'homemade' clothes, either- I do specifically remember that.
Fast forward twenty-some years, and I've sewn clothes for my niece. My sister & I buy matching and coordinating clothes for the babies, and occasionally, we call my daughter & my nephew the twin cousins, as they were due 13 days apart- spoiler- they were born 27 days apart. Still in the same month, but one will be celebrating their second Christmas this year and the other, his first. :)
So what is this sickness? Is it some momma rite of passage? Is it just innate to match young children?