So even fmc’s mom can inherit her husband if he dies ? What a shitty society

Thanks for the tl
Depends on the skill set of those left to take up the mantel. I'm sure all sorts of exceptions can be made, but if a household can't fulfill their duties, they would cede their responsibilities to the crown to be reassigned.
Currently the family disposition is a weaking lord and his wife of unknown health, an almost adult daughter, and the MC who has nearly aged out. In cases like this, the crown could do anything from taking a wait and see posture, assigning a suitable spouse from amongsthis retainers, or just demoting the family and recovering the land.
But none of that matters. The MC found an unlikely partner on a random balcony with nothing but a bottle of wine, a shoulder to lean on, and a shared gripe with their current life.