"His sister is in prison."
Translation:Hans syster sitter i fängelse.
In many cases, the Swedish verb sitta is used when people or objects are 'stuck' somewhere. If you glue something onto something, we will say that it sitter, or sitter fast afterwards, for instance. So the Swedish sitta has a strong flavor of this, and his sister as a prison inmate is certainly stuck where she is :)
That's interesting to know, but the dictionary hints offer just fängelset as a translation on "in prison", not i fängelset. Since it doesn't sound as though anything along the lines Hans syster sitter fängelset is grammatical, I'm reporting a problem on the hints for this question (having gone through the practice several times until I got it again, to check it said what I thought it did, and take a screenshot...)