Indeed. To humans you're considered old at thirty (pah!) and if you're not married by then, time to invest in some cats because you'll never get married!
Maybe not, no. Though to me, to peg them as siblings and have it run that way would be odd, in itself. At least to me. Though I've heard of it before---hell, I've heard of actual siblings getting married and that's even weirder, because they're related by blood. Weird in a manner of speaking, beyond quite taboo.
Although.... regarding this... world, this city and this culture, fictionally, nekos don't live as long as humans. I mentioned that by their thirteenth season they're rather independent, and not long after they begin the search.
In the states it's not legal, I believe. And I grew up most of my life there, unfortunately. Still, I do feel a bit weird about it all. And I don't usually feel weird about much!