Our girls are a few weeks shy of being 7yrs apart; both have B-days in June. Right now our oldest is 10 with our youngest being 3. We've planned to all wear sorta matching shirts this year; we're each wearing a t-shirt with one of the Pixie Hollow Fairies on it(except hubby he's Peter Pan) with our youngest also wearing a green tutu that I made her cuz she's Tinkerbell, oldest is Fawn, BFF is Silvermist, DSis Rosetta and I'm Iredessa(sp). Now, I did ask her if she wanted a tutu and she rolled her eyes and said "I'm good mom".
But I remember back in 2007 when the oldest was 8(nearly 9) and our youngest was nearly 2 and they matched a couple of days no problem. So to answer your question she doesn't mind matching with her baby sister as long as you don't make her wear a tutu.
T.