@Kazumaik the manga skips a few things (mostly the backstory of her brother and a little bit of world building). The part where she sleeps with her father and brother is when she "reconciled"/or to be precise solved a misunderstanding with both her father and brother. Minor spoilers (though they can still show this later but the one who injured Roux was brother who if I remember right was merely training on his maternal side of the family's assassination/ninja techniques (the organization where their grandma is the leader or is a high ranking official).
Father after his wife died overworked himself (since he was the minister of finance, apparently the royal family trusts him so much since despite being a count , his job and to an extent his blood as a mage (both siblings are talented as mages, Sera's mana pool is just bigger than her bro and papa) + his wife's combat lineage make their family well known in the noble community. Sera attempts to bond with her father by consoling him that she's fine she hasn't met her mom, he and her bro are still here for her
Laruzza has misunderstood Sera and thought she hated him (he feels guilty she never got to experience their mom's warmth, their mom's last wish is ofc to take care of Sera but again the brother feels like he isn't enough to care for her/lacks a woman's touch and just avoids her, Sera also misunderstands his behavior. The relationship gets worse when he realized Sera made a cotract with Roux and that he was the one who injured Roux. He thought Sera did the contract to spite him. The misunderstandings are solved and they spend as many nights sleeping in the same room (since brother has to leave to the academy eventually and their father still has work as literally the kingdom's treasurer. But again, the sleeping in bed thing here is most likely when the problems in the family have already been solved). After all this both men eventually adore Sera even more.