I'm Not Serious Yet

Weird development where the main character, who is the fourth son of a noble household and is called the stain of nobility, has to inherit the house because his three brothers were conveniently in the same party and all died to a meteorite apparently. The older sister who's left disowns herself and instead registers herself as his foster daughter and now she calls him dad all to get him to head the house and have him demonstrate his true ability.

Note: Includes a prologue, and six bonus chapters