The French medical system should butterfly away Margaret dying during her first birth like OTL. Margaret would be the name for the first daughter based on her mother and both grandmothers sharing this name. Blanche is likely to be the second daughter.How's this sound:
Louis X of France (b. 1260) m Margaret of Scotland (b. 1261)
Issue:
Louis of France (b. 1280)
Alexander of France and Scotland later Alexander IV of Scotland (b. 1282)
Mary of France (b. 1286)
Margaret of France (b. 1288)