Which part of it?

(The answer is probably 'yes' but she needs to make sure.)

Well, you can't really separate things in discrete parts. There is the fact that when their parents died the thaumatophobic doctor decided to call a poorly trained thaumatophobic security guard who decided it was a good idea to point a gun at Felix's head in case he reacted badly to the news "your parents are dead". That is by far the most absurd part of the entire situation and even made the news. The United States government even made a point of looking very apologetic about it. Then their grandparents took their younger sister to the Stormlands and decided not to send her back and kept insisting that the twins go against their parents' wishes and go to the Stormlands too. The government was suddenly less apologetic and just absent. The twins' parents had named their surviving uncle and (a fiancée at the time of the writing of their will) the uncle's ex-girlfriend to become their legal guardians upon death. The uncle moved back to the Stormlands after the break-up and the twins had to convince the ex to let them crash at her place while the twins got emancipated. They suspect, but have no concrete proof, that their grandparents have been sabotaging the twins' attempt to work using their skymagery.

...she did not know about the sister part. This does not make her happy.

What would an acceptable outcome look like, to them?

In order from the most to least important:

1. They want their sister safe.

2. They want their sister to not be forced in an arranged marriage for the purpose of skymage-gift eugenics.

3. They want their sister to not be under constant pressure to be a skymage dedicated to serving the community.

(Oh, they just remembered that this correspondence should be confidential. So they're free to reveal that their uncle (the gradparents' eldest) killed himself because of emotional pressure from the maternal grandparents. They wanted him to be a skymage dedicated to serving the community.)

4. They want their sister living with them.

5. They want their grandparents to not influence the twins' job prospects negatively, if they are doing that.

(As a side note, they explain that they have family from their father's side, but they are more or less estranged and that set of grandparents is likely to do what the other tells them to. So putting their sisters with them is not an option. Their surviving maternal uncle is not available for the same reason.)

Define "safe." Do they expect their grandparents are physically or emotionally abusing her?

Not physically abusive, no. The grandparents had high standards and their children would need to earn being allowed to have fun, seeing friends outside of school hours or even having time with their own parents (on the grounds that their time was too valuable and helped many lives). Their mom remarked that their dead uncle used to get a lot of praise and the other was largely ignored like he wasn't there. She was somewhere in the middle. Allegedly the grandparents have tried to improve over the years, but they took their sister and didn't let her go back.

Okay, this she can definitely work with, even if it wasn't exactly what she was hired for. The biggest problem is that they're still minors, but if they can prove that their sister is more suited to being raised by them they can get custody that way. It's going to be extremely hard, though, the system is rigged against them.

They are grateful! They didn't even have better options before. They even considered going to the Stormlands and then share the burden of being around their grandparents (they didn't went for that because it would be so much harder to actually rescue Maira from the situation.)

She can start looking into that, then.