Elli, I think it is a shame that you feel like that. My grandmother didn't like me or my younger siblings. In fact she only liked my older sisters and my older cousins. I knew it when I was quite young and I knew that she also didn't like my brother and sister who are twins and nearly four years younger than me. I always knew and it was quite obvious.
Interesting that all those years later when she was old, I was one of the very few of her ten grandchildren who ever visited or took her anywhere. The same thirty years later, I and my oldest sister and one of my cousins (who is the same age as me) are the only ones who visit my aunt, who was very difficult when we were children and who has no children of her own. The others who were so favoured are too busy, can't be bothered, don't care.
I might add that having a seven year old staying when you are not used to it for two weeks is probably quite tiring. It might not be her it could be her age. I think nanapug is right, try and have special time with her and get to know her. My eldest granddaughter is six and a half, what a fun interesting child she is. I think it is a great age, they are not babies but are so much fun and see things differently to us. At the moment I am teaching my granddaughter to touch type and I listen to her read, I have made sure she knows how to hold a pencil correctly. I am even teaching her how to whistle with two fingers. If I could knit I would teach her that too, all the things her mother doesn't have time to do, or even think of. My GD loves learning new things and she wont be this little person much longer.