It’s very odd, but this almost certainly has to be There Once Were Stars, by Melanie MacFarlane.
- Female protagonist.
Lives with her grandmother:
The clock on my dresser reads eight o’clock in the morning. It’s been
nine years since I moved in with my grandparents, and saying
Grandmother and I have differing opinions barely touches the surface.
Her rules are sometimes worse than those of the Order, who police the
dome. With any luck, I’ll be assigned my own apartment soon, and will
finally be able to restart my life again.
In a dome:
I trace the row of stitches, squeezing my eyes shut as I make a wish;
it is my eighteenth birthday, after all. But when I open them, the
same scene shows from my bedroom window that always does—the grid of
our dome. Nothing changes. It doesn’t matter how many birthday wishes
are made; I always wake up trapped inside the dome. The grid of thick
glass and steel arcs far above our apartment, stretching to where the
great Axis, a tower of government offices, meets the peak of our
home—Dome 1618.
And she sees a light outside the dome, where no one should be (admittedly, not at night):
Another light flashes, but this time it’s in the distance, on the
other side of the glass. I lean forward, focusing on the light, and
see a shadow move on the other side. My entire body goes rigid, and my
heart beat thunders in my ears. No one could be out there—unless—could
it be an Infected?
Because of radiation:
They were all killed by the Cleansing War—everything was. If the nukes
didn’t kill them, the nuclear fallout afterward would have.
And the protagonist finds her way out of the dome.
And she stays behind while the protagonist escapes with a boy:
We make our way outside the dome. When I feel the open air against my
face, I know I’m finally free. The night sky stretches out above me.
The moon and stars light my way toward the life my mother wanted me to
have. I turn and see other people flowing out of the open dome. In the
distance, I can hear music in the air. People are celebrating.
I look out into the skies of the mysterious beyond. My fingers reach
over and entwine with Evan’s. I whisper, just loud enough so he can
hear me.
I think the poster might be conflating it with Pure, which visually has the description they mention:
It was published a few years back, and is in a series of books with simple titles. The element of some relative falling in love with someone outside the dome might have come from there (misremembered).
However, the first book was published only six months before this question, which seems like a very short time both to read it and forget enough conflate it with another book.
