I haven't heard of "instairs".

Google translate of French is "go between the stairs".

@needs native check

The French sentence seems to mean something like "Enter by the staircase" or "Use the stairs to come inside" or something like that.

Looks like "instairs" should have been "in stairs", but it doesn't make much sense either.

Yeah, it looks like this sentence means "Come in through the staircase". "Instairs" and "in stairs" don't make any sense

Ah, actually the French sentence was an attempt to translate this English sentence, not the other way around.

I'd suggest deleting this sentence if it doesn't make sense to native speakers of English.

Oh. In that case, I think the sentence's intent was to be weird and be like "Ah, you see, it's like the words 'upstairs' and 'downstairs', but the catch is 'instairs' isn't a thing! I'm hilarious!" Anyway, this isn't really translatable at all.

Native English speaker. This is not correct. Although, it is so novel that if spoken it would be endearing and possibly provoke laughter. All in all, it was enjoyable to see this.