I followed the trail of links from your AO3 yuletide gift all the way here, and wow! Irulan definitely gets shafted in the books, and thank you so much for putting the picspams together!

Re: Wensicia being so much older - Irulan is the eldest of Shaddam's daughters, but living on Arrakis gives a natural level of exposure to spice, which apparently slows aging? And Salusa Secundus is meant to be super severe and not a great place to live, so. *handwavey*