"What was her last wish?"
Translation:Vad var hennes sista önskan?
I'm not a native speaker, but I think 'senast' means latest, while 'sista' means last. I imagine you'd use senast when you're speaking about something that is temporary ("the girl's latest wish is to buy a zebra") and sista when you're speaking of something ultimate ("last will and testament").
Objects that are 'owned' by another are treated as definites, so sist has to be sista.