I hate my best friend's little sister and she loves her!?

My friend and I are 12. Her sister is 5. My friend is called Mel, her sister is Jessica. They moved Germany, so they sometimes come to visit us. Our parents are good friends and our mums have known each other since primary school. They normally come to visit us for a week or so 3-4 times a year. Mel is my best friend, I have known her since we were born! Jess is really spoilt and because the whole family love her, she gets away with anything. Whatever she does, she never gets into trouble and they always forgive her. Jess can walk into my room when I am not even there, she doesnt think it is bad to do that, it hasn't ever occured to her that maybe she should ask. If I ask her to leave, she acts really offended and cries. All I say is 'Jess, please can you leave my room. Next time ask me please', which isnt mean at all! She also slapped my cat once (she was playing with him). I asked her to be careful, but she ignored me. After I saw her poking my cat's nose, I said 'Jess, I dont think he likes that. He told me that he doesnt want to play with you right now and would prefer it if you just spoke to him or looked at him, but not touch him. Is that OK?', she said 'Yeah'. Now she still does everything but hides it from me. If I see her and ask her why she does it, tell her my cat doesnt like it, she gets upset. She sometimes takes my toys or my stuff and plays with them, which is totally fine by me, but then she goes and starts taking them apart, or plays with them violently and breaks them. Her parents saw this once, but they just said it 'Jessie, dont do that, sweetie', then when she broke it, they just said 'Oh, Jessie, look! You just broke it!', in a calm, sweet tone. Her family dont seem to have a clue that I dont like it when she walks into my room without permission, slaps my cat, breaks my stuff, lies to me etc! What should I do? I dont want to tell Mel, as she wont like it and it might ruin everything because she loves her sister and would probably not like it if I said that I dont want Jess to play with my stuff or follow us around. She would just say that Jess is little and really cute and there is no point telling her off. My parents just say 'Well, I cant do anything except to ask them to visit less, so unless you want me to do that, there is nothing I can do', I want to still see Mel, as she is really nice and completely different from her sister. She is a really good friend, just her sister turned out really spoilt... I cant tell her parents, because they will stop liking me and think I am making a fuss! What should I do?