That's probably for the best, if you even look at any girl outside of the household at any time, you will get locked into the housefire ending with everyone you did care about except one dying horribly. Because dramatic reasons.
Alright, that was a piss-poor attempt at april's fool, and I'm no good at it.
Truth is, you won't be punished for being close with any combination of characters in LomL ever, and no-one will end up with hurt feelings. Plus there's some fun acknowledgements,jokes, and extra conversation options if you do go after the full set. So feel free to go for whatever combination takes your fancy.