Why does my uncle’s girlfriend hate me this much?

I am 18 and they’re both, 35. I’m very close to my uncle, and I do feel like she see me like shit. For example he invited me to have a dinner with them, and I heard them behind a door fighting because of that. She ordonned to chose between me and her. To stop that, I waited them to come back to the living room, and told them that I had to go somewhere else. She seem to get along with my brother , but she hate the fact that I’m my uncle’s favourit niece. I don’t know why she is acting like that, I mean... it’s Clair that my uncle and I, have a normal relationship, and that if he don’t see me like a niece , he see me like his daughter or his little sister. But his girlfriend even act like this with me at home, for example she bringer à présent for my brother and my parents but she acted like she forget my present, or during my birthday, she created a problem because my uncle offered me an expensive necklace, she yelled at him, telling that spoiling me will turn me into a bitchy brat. I don’t want to creat a problème so I just shut up, and I even stopped to visit my uncle on his house like I was doing it, or when he went to hang out with me, I find an excuse. What should I do? Why is she acting like that?
Why does my uncle’s girlfriend hate me this much?
Why does my uncle’s girlfriend hate me this much?
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