"My sister works in a prison."
Translation:Søsteren min arbeider i et fengsel.
"My sister works in a prison" can not be translated into "på et fengsel", as you don't say that in Norwegian for describing that you're employed at a prison. However, I have a prison nearby, and those working there "jobber på fengslet" (fengslet almost gets a capital F here). Are there any jobs available at the prison? Er det noen ledige jobber i fengslet?
You can be both i/på butikken (at the store), but working there "jeg jobber i en butikk" or "jeg jobber i butikk". Or "jeg jobber på butikken". But not "på en butikk", then it suddenly turns into construction. Some different workplaces: På en skole/et sykehus/et sykehjem/en båt/en kirkegård/et verksted. I en barnehage/et fengsel/en kiosk/en butikk/en veldedig organisasjon/en kirke/en forretning.
Prepositions have some rules to them and a lot of exceptions. It's just one of those things that are not logical. You have to learn them.