It's doable, I bet it would get tricky to manage though. Just remember what day was supposed to be the younger sibling's birthday, after the older sibling's classes for the day go to Manage Worlds, choose the original household, blow out the candles, and enroll.
The tricky part will be managing two sims as their terms end on different days. It will be very easy to forget to submit term papers and presentations, or study before finals. It would be a lot less tricky to either: age up the older sim 4 days early, now making them exactly 1 week apart in age, and now they have the same schedule just 1 term apart, Or, the older sim just wait till the younger sibling's birthday and they go off together.