I wouldn't worry that the programme is so old. It does everything that the newer ones do. I have tried and used/use Adobe Photoshop, Photoshop Elements, Jasc/Corel Paint Shop Pro, Serif Photoplus plus others and I always have gone back to Photodeluxe. It's what suits the user and your sister is the user, not you. The simple interface is indeed simple, it's what makes it good in my opinion. I bet me and your sister know where evrything is, what everything does and could do just as good a job at anything with any current programme other than maybe full blown Photoshop. A million buttons on view to choose from when you only use 10 complicates things. Your sister will return to the programme whatever new one is tried. New doesn't mean better. I salute your sister for not bowing to the onslaught of pressure to get the new fangliest, shiniest, buttons R us, lets complicate things programme, when the one she has already does it all. Long live Photodeluxe, resistance is futile, they got it right then, so there's no need for change.
Of course, new computer and no disc doesn't help.
None of the newer programmes 'feel' the same to use, but if you do a search on this site for 'Paint shop pro 7' and 'download', you should find a link to a free download of this programme for her to try, it's as close as she'll get to something she's comfortable with, but it'll never be the same.