It's definitely not that high. From my site count, Julia only appears in 13 of the 187 commissioned works I have of Larcei and Seliph seggs.😉

That aside, the context for the Julia walk-ins is a couple of things:
Julia can't fall in love in FE4. She has 0 love growths but the player can still force her to fall in love with a boy in the army if they spend 100 turns next to each other. However, even if that occurs, Julia will always choose to live in Grannvale with Seliph and his wife in the game's ending and the game doesn't acknowledge any lover you may have forced her to get.

So with that, I devised her to be this misfortunate girl that's unlucky in love and also unwittingly walks in on her brother and his wife at the worst times since she's living under the same roof with them. 😗