In the AM, immigration at MAN is okay. Small queues but still count 15mns minimum.
Security on the other hand, long queues and long process times. Unless you have a premium ticket you will want to buy a "fast track" or whatever they call it.
Finally since you made the right call and extended your ticket, there are lounges which you can book for a fee (again, if you dont have a premium ticket) so your sister can take it easy.