Thanks for the chapter!
LN gives more details. Felis is that cunning, psychopathic, sis-con prince's sister who's always sickly. Although I have doubts about whether that's because he almost never lets her leave her room, is actually sick, or just pretending to have him dote on her or some combo of the above. MC's crime was discovering and not understanding the prince's love for his sister. Although the prince wasn't exactly subtle about it such as suddenly canceling meetings with MC when she came back briefly from war to be with Felis who wasn't feeling well including the MC's birthday. Lapdogs the prince mentioned are the division she led in the war and it's suggested he killed them all off to prevent any protests about the treatment of the MC. House of Saville is the noble house she was born into as the head of the household's daughter. Can't remember if killed them too but in this case probably arrested or imprisoned them under the trumped up charge of treason and conspiring with MC for her "crime".
Spear he threw isn't an ordinary spear but said to be the spear the goddess the country turned into to defeat some supposedly evil OP guy in another country. Not sure if actual spear goddess turned into or a fraction of her power turned into a spear granted onto prince that the goddess can control and observe things through. More info about that should come up in later chapters so won't give more details. Same thing with the snake like thing she saw that disappeared.
Pretty sure they had locks back then so why didn't the prince make sure to lock the door before doing the horizontal tango with his sister? Guy really is smart and cunning as was able to lock her up and sentence her to execution with a few days of discovering his secret along with holding the trial without her. Btw, there's a reason he's ticked that she's not accepting his proposal other than just pride.