Actually mostly it's Locative and it is Locative here. But it does depend on the function of 'w'. English wiktionary states three most important meanings:
- (+ locative) in
- (+ accusative) into, in
- (+ accusative) on (time/date)
For example, to differentiate between two first ones: if something is literally inside the wall, it's "w ścianie". If you hit the wall, that's "w ścianę". Your brain is in(side) your head, that is "w głowie". If you hit yourself on the head, that's "w głowę".
I know "swoja" has not been explained in the course yet, but...if we really meant: "We visit Our sister in July.", shouldn't we say then (as long as we don't want to use "naszą"): "W lipcu odwiedzamy Swoją siostrę"? Even Google Translate translates "W lipcu odwiedzamy siostrę" as "We visit My sister in July."
Thank you! Then, if I am not mistaken: "W lipcu odwiedzamy siostrę." could (depending, of course, on the context) perfectly be translated into: " We visit my sister in Luly.". I think I can conclude that also from other comments. And I completely agree with what yoy say bout GT, of course.