My uncle is acting strangely with my sister

To be blunt, i noticed that my uncle, (49M), has been paying a lot attention to my sister, (14F). it's not just that he gives her more gifts or just prefers her, but whenever we show up to his house for a family event or just something similar, he always talks about how she looks and is always doing stuff like patting her lower back, rubbing her shoulders, or he'll grab her waist without warning, his gifts for her are also more expensive

it was fine at first, he was at my sister's birth and was one of the first to hold her, so shes always been his favorite to an extent and hes always spoiled her, but its like i blinked and hes suddenly acting like this. i dont know exactly when he started acting differently with her but i noticed it a week ago and we havent been to his house since then

ive tried talking to my grandma about it but she just brushes it off and my parents are always too busy, im also scared of telling our other extended family because of the fact my grandma brushed me off the first time. what should i do? cause i dont wanna just ignore it but im also scared of telling anyone else incase im overreacting

EDIT: this isnt really an update but for the comments who asked, i havent talked to my sister about it yet but im planning to do so later, we're at our grandma's house right now and considering our grandma brushed off what i said, i dont want to cause too much of a fuss if she ends up hearing me bring it up again

im also planning to talk to our parents as soon as we get home, all your comments drilling it into my head that i had to tell them helped, thank you for helping me not be a coward and for your guys' support in general