Is AUTHOR wrong in the post ? POST : My gf Lara grew up without parents, but with a big inheritance. There was a non-relative woman who took her in and who became her mother figure. This quasi-MIL seemed nice to me, but behind my back, she was telling lies to her entire family, like that I never paid rent after moving in with Lara and that I only liked her because of her money. She started trying to convince my gf to leave me. Lara didn't tell me about this for months.

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MIL claimed that Lara has become like her second daughter, but while she always made sure that her real daughter had enough time to study, she never gave a shit about Lara's education. Lara instead was asked to do an unappropriate amount of chores at home, that literally kept her from studying. Lara didn't even get to go to the same prestigious school as the real daughter did. Still, Lara felt thankful to have a family in them that she even paid them rent and bought their groceries in order to not be a nuisance. So these things Lara confided in me. It was her own idea to pay the rent and groceries, and she never saw anything wrong with doing all the chores. She never noticed how weird it all was.

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After months of MIL's campaign against me, Lara had a long conversation with her quasi-mother, telling her she'll spend the rest of her life with me, and if MIL doesn't accept that, she'll only play a minor role in the life of her future quasi-grandchildren. From then on, our relationship improved a little.

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Then, a friend told me what had been happening behind my back prior to my gf's conversation with MIL (which I didn't know about). It sounded like it was still going on. I asked Lara about it, and she confirmed that MIL had said and done all these things. I wrote MIL and told her I knew about her trying to split us up, and about the rent lies. I said that she's a hypocrite to say that because she didn't treat Lara like a second daughter, she treated her like a slave.

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After my rant, MIL just blocked my number and sent Lara screenshots. Lara was angry , she said that the arguments I used were things that never had bothered her herself. Also, she told me about the conversation she had had with MIL, and that our relationship had improved so much already, and that I had destroyed that. MIL went low contact with Lara until a small unrelated fight, when she broke off all contact. 

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Sometimes I wonder if I should just have not said anything at all to MIL and just not have come along anymore when Lara visited her. I'm responsible for her losing her mother-figure, even though it was a horrible one. Without her, Lara probably wouldn't be the amazing loving person that she is. Lara isn't angry with me anymore, she has realized that while it was not my fight to fight, what I had told MIL was actually right. 