What do you think Eponine and Cosette sacrificed for Marius that showed each girl loved him?
To speak toward the comparison of Eponine to Cosette, I'm afraid there is no comparison. Cosette sacrifices nothing. She is but a "lark" whom Valjean protects so completely that after a few years in a bitter childhood, she never wants for anything again. She doesn't even realize the agony she puts Valjean through in her love of Marius.
At the end of the novel, I didn't like Cosette. I understood the point of her character, I pitied her childhood, I even triumphed in the victory of Valjean fully saving her. But in a comparison between Cosette and Eponine, Eponine is by far the stronger more noteworthy character. She too, had a pretty bitter childhood, but the difference here is she was not rescued, so her childhood carried over to an even more bitter life. And yet, instead of acting like a victim, ever, she dies for the man she loves. I don't think she did it to be with him in heaven. I think she did it because she knew there was no other way for her to show how she felt. She was never going to get Marius' attention alive. She knew that. But in her death, she secured the fact that he'll never forget her.
Wow -- that's a lot of detail. Let me start back at the original question: What Eponine has sacrificed for Marius. In essence, Eponine puts aside her own feelings so that Marius may pursue the person he truly loves, Cosette. She keeps her affections to herself so as not to interfere in the Cosette-Marius relationship, all the while longing in her heart after him. In fact, upon her death, when she is being held by Marius, she declares that she feels perfectly wonderful for no other reason than because she is in his arms. In the Broadway version based on the novel, this is where the song "A little fall of rain" comes in:
"...a little fall of rain can hardly hurt me now,
you're here, that's all I need to know,
and you will keep me safe,
and you will keep me close,
and rain will make the flowers grow."
The biggest sacrifice Eponine makes for Marius is that of her own inner desires.
Eponine gives up her very life for Marius. This ultimate sacrifice demonstrates her true love, unselfishness desire for another's happiness. Courageously, she rescues Marius on the battle line and becomes herself a barricade, one between Marius and the bullet that kills her.