basically, the MC's parents invited a bunch of nobles and wealthy heirs to a party to celebrate their daughters birthday, then made the other daughter the center of attention, to the point of buying her a new dress, and the only thing the MC got to do special was use laundry detergent to wash her hair and is wearing a very plain (and I think old) dress, and made it look like the preferred (and older daughter) was the center of attention and the birthday girl to anyone who did not know their names, though it seems that the sister tried to object and was silenced.

basically the MC leaves "her" birthday party, and no one notices or cares.

She mentioned in her narration that the family is pour, and trying to marry their daughter off to a wealthier noble for an influx of resources.

After leaving she runs into a late arriving guest who mistakes her for a servant, before realizing that she is a daughter of the family, they have a conversation and he says they should talk more sometime, but she shakes her head as it is apparently forbidden by her parents.

She ends the conversation and leaves, apparently the MC is forbidden to discuss allot of things including that she has to attend a school for non-aristocrats, as well as other things for fear it will hurt her sisters reputation and marriage chances, she directs the man to the hall and the star of the evening (no names mentioned), goes to her room (which is a shabby servants room) and cries.

About a month later her sister gets engaged to a man that they do not know, and do not remember ever meeting

the MC visits her sister in her room during the, apparently hellish, bridal training. sisters room is about 3 times the size of hers, and filled with much higher end furniture and clothes then the MC's. the sister seems to be stressing out over the upcoming marriage, the fiance seems to have a reputation as a woman hater, and they have never met, so she has no idea what he is like. With the way this chapter seems to read it would not surprise me if the sister faked her death to escape a marriage she as afraid of, she even shows the MC her secret (from their parents) hobby, and confesses what her plans were if the family fell into ruin. Which feels like foreshadowing to me.

The sister is no where near as bad as her parents, but I an not certain she was a good sister from what little we see of her, and even seems to give the MC some words of encouragement.

Her reflection ends, and the MC is back in the carriage to her sister's fiance. with her father ordering her around, complaining she is not cute, tells her to not stay in the capital and try to find work once the fiance gives her the boot, and even ends with saying he wished she was dead.

it is not that she had none, but her parents neglect and abuse has lead her to not being able to see value in herself, she has been told she is ugly so much she doe not even look at herself in mirrors, and does not recognize herself after being forced to properly bathe and clean herself since her family has not let her even get properly clean as opposed to themselves which caused some appearance issues like the hair in the description, on top of her family using her as their servant (getting up earlier, doing most of all of the house work, and it seems her meals where whatever scraps they left on their plates), plus is seems she was doing the paperwork for actually running their expenses and territory

Very predictably he is the man she met after leaving the birthday party, because no names were mentioned and when her arrived at the party her father directed him to her sister as the star of the evening he mixed up their names and got disgusted with the family, I assume there are some broken rules of etiquette involved and the parents were supposed to make sure to introduce everyone who asked to the person whose party was named on the invitations as the reason for the party (the MC not her sister) As a result he thought his proposal was rescuing an abused woman from a horrible family. But he was also interested in her before they parted that night, as it seems her mind impressed him (his mother if a foreigner, and she was the first person he met able to guess his ethnicity, as well as being fluent in his mothers language) and it seems could see signs of beauty hidden under the dirt and grime. No taming the beast or brutal woman hater, she accidentally won his heart with words alone. Also, he outright asks her is she was abused very early.

Since they didn't find her corpse, and we all know that no corpse = no real death. I think she is alive and just escape from the marriage.