Brain-Teaser 512

From The Sunday Times, 4th April 1971 [link]

When I first met the Problem Setter, and let on that I was interested in his line of entertainment, he immediately told me that he had five nieces all going to the same primary school where each girl was entered on her fourth birthday and on each subsequent birthday morning was moved up into the next form.

Forms I-III were the Infants section to which they all still belonged. Tomorrow (Tuesday) one of the five would have a birthday, on Wednesday another. Which?

To get me going, he gave me their names in order of seniority and told me which niece would be in which form on Wednesday.

I said all I could usefully deduce from this was that Alice wasn’t one of the celebrants; I couldn’t be at all sure of any of the other four; I was aware, needless to say, that Alice, though younger than Emily, was older than Isabel, and that Olive was older than Ursula, but would I be right in guessing that Wednesday’s celebrant was older than Tuesday’s? (This necessitating that Ursula and Olive were currently in the same form).

When he said yes, I knew the answer, so …

Whose birthday [was on] on Tuesday, and whose [was] on Wednesday?