Eponine felt bad at first for what she was doing. Cosette was nice enough to allow her to stay with them when she left her parents' home. She was even helping her get an education. It wasn't like she even deserved Cosette's help after everything she did to her when they were children. Cosette was big enough to forgive her though. She didn't think Cosette would ever forgive for this. This was so much worse than bullying her as a child. She loved Marius though so she couldn't help herself.
They were messing around right up under her nose. Whenever Cosette went out, she and Marius would do it. It started off with simple kisses then eventually it became the ultimate betrayal. They would also mess around when Cosette fell asleep which was usually around the same time every night. Marius loved her and if this was the only way she could have him then it was fine with her.
Despite all of this she was becoming very good friends with Cosette. Cosette would talk to her about how lonely she was her whole life and she how she still was. Having Marius didn't always fill that void. She had been sheltered her whole life and all she wanted was to be free and be able to have fun like other people. Having Cosette confine in her made her feel even worse especially when Cosette told her that she was her only real friend. She also really liked Cosette and felt the same way about her. She loved having a girlfriend.
Cosette wanted to set Eponine up with some of Marius friends even telling her that she and Enjolras would be perfect for each other. She didn't want Enjolras though. He was cute and she loved his passion but she only had eyes for Marius. She would never tell Cosette that though. She felt bad about it but not bad enough to stop carrying on with him.
Marius loved her. He told her so. He was honest with her and said that he had always loved her but never thought they could be together. Plus socially he was expected to marry a girl like Cosette so he always tried to forget Eponine but he couldn't. He told her that it was like fate for him when Cosette asked Eponine to move in with them after the wedding.
Eponine was surprised by what had happened next. One night Cosette had come to her. She was crying. She told her that she thought Marius had a mistress. She was very afraid that they had been caught then relieved when she told her that she was sure that Marius was paying some prostitute in the streets. She let Cosette cry in her arms that night. The next day Cosette told Eponine that she was happy that she would always be there for her and said she was very pretty. Then she did the unthinkable and kissed Eponine.