I made the mistake of taking Jostein as LI as well and I regret it a lot, because it is surely the worst route in terms of romanticism. I just started chapter 5, and I haven't felt anything with that route, for me Filipa isn't the problem, since she's super adorable and treats you like a sister, it's just that you always feel like you're the mistress, you're literally going to be the one who gets saddled with the worst part of marriage, which is housework and taking care of the kids, while they go on their honeymoon and spend it making more babies. Honestly Jostein will be as handsome as he wants, but his route is terrible and I really regret picking him as LI, they could have simply set it as a supporting character who shows his dick from time to time and I would have felt the same way. I don't know what will happen in chapter 5, the save or let Filipa die thing I knew it because I saw it in the guide, but I haven't inquired about the development of the relationship neither chapter 5 nor in chapter 6, but I don't have much hope to see anything either. If it wasn't for all the grind that the game requires to advance I would have already changed my route, but honestly I've been playing for about 60 hours, and for what's left I'll finish what's left of the route.
Literally the relationship consists of being the family maid, super romantic if you ask me.