The Eldest Daughter, Called Slow, Plain, and Useless, Is Doted On by Her Magic Master

Why does FL think she needs to run far away? A year ago she was offered a job as a royal mage. She could absolutely find work as a mage anywhere she wants.
The mage tower only allows opposite gender apprenticeships if they're explicitly romantic? That is some predatory bullshit and ML isn't beating the grooming allegations.
Always a great plot point when the dad insists his daughter needs a mother, so he quickly marries the first lunatic he sees and abandons his daughter with the woman for the next 15 years to do business (clearly not very helpful business mind you).

Her hair turned from silver to brown and FL just forgot? And not a single person ever mentioned how weird it was? And ML noticed but only just now decided to bring it up?

ML is a groomer.
He meets FL when he is an adult (unknown age) and she is 8. He decides to make her his apprentice after confirming she has no responsible adults in her life, and only meets her in secret.
It is later revealed that he's distantly related to her and knows about family that wants to see her. Despite that and his own high status, he does literally nothing to try to improve, protect her from, or explain to her good family members anything about her the abuse she faces at home. This leaves him the only emotional support in her otherwise bleak life.
When she leaves her household at 18 and he finally does take her away, he teleports her to a place she's never been and can only rely on him. Only then does he say to stay there, she has to become his fiancee because the mage tower has incredibly predatory laws about apprenticeships.
FL has unconsciously used magic to turn her hair brown. Despite this never being a problem before, and FL having met plenty of mages over the course of her life and education, ML tells her that other mages will be insulted by her "concealing" her appearance with magic. There is no suggestion that they could just explain the situation the 18 other very friendly mages in the tower. Instead the only non-painful option is for ML to temporarily dispel the hair color change by making out with FL. Which of course they'll have to do on a regular basis...for FL's reputation of course. The first time he starts making out with her, ML does not warn her that is what he's about to do, so it's just assault.

The overall story is standard, really. Stepmother hates her, little sisters lie about and isolate her, whole estate rides on her back, fiancé can’t keep it in his pants, then ML swoops in, whisks her off, and heals her pain through the power of love. All the abusers in her life have their worlds fall apart because she was the one holding shit together.
The interesting parts have more to do with the character settings for the MC and ML…but neither the writer nor the artist has the chops to tell this story in a way that isn’t creepy. The meeting when she was a child where he became her magic teacher, his wanting to whisk her away once she grew up, their familial ties — you have to pay close attention to realize it isn’t grooming (it isn’t, guys, their relationship would have an entirely different MO if it was), and you have to be comfortable with the fact that second cousins are legal in a lot of places. You also have to reconcile the ML taking the MC as a fiancé in a way that was somewhat disingenuous since he didn’t make his intentions clear. 
The art is cute at a glance but has no emotional weight, so it simply can’t make up for the shortfalls of the writer.
I’m not going to say it’s a bad story, because there are points that I like, but know your triggers going in and see yourself out if you don’t want it.

The twins and their mom are absolutely heinous 

This *really* feels like ML is grooming FL. He became her mentor when she was 8, and at 18 he wants her to break her engagement to stay by his side? And we 100% know it becomes a romantic relationship because he's the ML?

This sure is some Sweet Home Alabama shiet... Not only ML have been FL's teacher since she was 8 years old and ML was who know how old, at best case 15 years old: They're second blood related cousins.
At first I was happy that this series said from the start FL was 18 years old, not 16 or worse case 14, but now I know why the told HER age right from the start. Trying to sure us, readers, that nothing happened before she was 18.

Also, can we just ban name Leticia and similar ones for the female leads, because I'm going to loose my mind if there will be another FL that sounds like wannabe fancy and cute salad.
Better yet, lets ban all the names starting with L for the FLs, just to make sure they don't keep growing like lettuces. I want next FLs to be called some version of a fancy and cute carrot! Or potato!

I was fine with this until they dropped the distantly related factoid, let's not be doing that even if it's technically likely to happen within nobility. This is fiction and fiction doesn't need to follow the same rules as actual real life nobility where most nobles are distantly related. 

Woah dad’s even more incompetent and unobservant than I thought he’d be. Like I knew step mom and the twins were garbage but damn dad why are you awful too

One of the twins is happy to become the mistress of the others husband? Well, I guess they're sort of humble?