What’s the name of the anime that’s about a girl coming from a star? The main character is two boys that’s friends and when the girl come from a star one of the boys transform into a monster. Later on another girl appear which is her sister and she falls for the normal boy. Throughout the show the boy that turn into a monster past keep popping up. Also the first sister ended up destroying the world.
I know you said "older" but 15 years counts, right? This sounds a lot like This Ugly Yet Beautiful World, namely:
two boys that's friends
Takeru and Ryou
when the girl come from a star
The on-the-nose-named Hoshino Hikari, who falls mysteriously from the sky as Takeru is riding on his motorcycle
one of the boys transform into a monster
Later on another girl appear which is her sister
Akari is a good fit for this, she appears in the same "falling from the heavens" type manner and considers Hikari to be her "older sister."
Throughout the show the boy that turn into a monster past keep popping up
This I can't precisely match to the series, but I wouldn't doubt it.
Also the first sister ended up destroying the world
Near-match, as Hikari decides to end the world, but is stopped by Takeru's love for her.
How I found it: I watched this myself in the late-2000s, and reading your question made me think of it.