what to do?

I am currently facing a situation that is beyond me. I have been in a relationship with a girl for more than a year and everything is going well, we are very happy and we live a pleasant routine. I am a very reserved person, who does not seek too much discussion, very discreet which makes me a little mysterious but not necessarily in the good sense of the thermal. I come from a family separated by 2 languages that I do not speak, which means that all my youth, I only knew my friends and my close family (4 people) which does not help social learning, or in the middle of discussion excluded because they do not speak the language. My girlfriend is the opposite, she grew up in a very large family with many family meals, a close family very open and understanding, while I am the opposite. Therefore, I may not be the one who inspires confidence, not by my facts but by my outward personality. Which brings me to my current situation:

When we were arguing with my girlfriend, she used to indulge in her sister and her girlfriend. She needed to talk about it and was confident because they were quite close. Disputes, even about immature and stupid and futile disputes, which is unfortunately the case for many disputes, have been reported. The problem is that they don’t know me like my girlfriend knows me or my friends, they don’t know me because I never really discussed my life and other things with them. The only times we saw each other was when I ate with them (rare), at the family meal, that’s all (they were not often at home). For them, I am the guy at the end of the table who does not talk, who does not smile, who is clean on him... a little creepy and scary what, which I can understand. But if we add the fact that we report our disputes to them, so that the negative in a relationship (which I specify is not at all unhealthy), they have made themselves an image diabolizing my person. Currently, they refuse to talk/see me, refuse my presence in their house if they are there at the same time. They feel insecure and embarrassed by my presence according to their statements.

A discussion between their family was made two weeks ago, where the sister explained the reasons for her feelings. The whole rest of the family did not understand her comments, contrary to what she said, they saw a positive evolution on my girlfriend because she was leaving her room more often, she made new friends, opens up to new experiences, etc... My girlfriend could not do anything or say anything without her sister is focused on her position and suffered the slander against me during the whole discussion. For some time, his sister lived at her girlfriend’s house only to avoid coming to the family home because I was there. Currently, she is claiming a turn because would like to come back home from time to time, but suddenly my presence must be forbidden by their parents, which they have accepted to allow her to return a few times, she remains their daughter. She even dared to reproach their parents for letting someone like me into their home and that they were not good parents because of it.

My girlfriend tried to understand, explaining by message her point of view on the situation because she is absolutely not in agreement with it but that has changed nothing, she remains on its position without wanting to understand anything. I asked for a letter explaining all the things that are reproached to me with examples and consequences on her "mental health" as she says, but at the moment nothing. Because everything I tell you comes back to me after (2 weeks), she did not explain the situation personally to me at any time, always behind my back.

I have a hard time understanding what is going on. I have a hard time imagining what could be creating that feeling. I have trouble knowing what to do about the situation, because all discussions are lost in advance, she says. I feel like I’m being whimsical, but at this level of severity, I really wonder if I should not go as far as filing a complaint. What do you think?

""Sorry if the text is not very clear, it is a translation via IA so maybe everything is not correct.""