Julia and I talked, and Evan is right as always. I don't think it's ever too late to plant sibs, unless maybe you live in the middle of Alaska. I had one a few years ago I was dividing on New Years Eve, with the help of my daughter. Got most of it replanted, almost 100 divisions, and it started to snow, so I just threw the rest on the ground. They were perfectly fine the next spring.
The only problem with planting late is you might not get as much bloom the following year.
Julia, I would suggest you get an early- mid blooming peony to get the combo you want, or a very late blooming sib. Or you could just go to the iris farm and see them together and get them. It's a wonderful combo.
My trading partner just sent me a peony Dancing Butterflies, and says it's a very early one that blooms with the tall bearded. I can't wait!