Is it normal that my grandpa is horny for my 17 year old cousin

My 50 or 60-some year old grandpa gets , like, really horny for my 17 year old cousin because my grandpa owns a pool and we go there often in the summer. She(my cousin) usually has a bikini on when we all go to the pool. Just recently, we were there and my cousin wasn't very tan, so she laid down on a towel face- down and my grandpa kept staring at her ass. I could see that he had a hard-on under his swim trunks. He was all like, "Hannah(my cousin), that's the whitest i've ever seen you!" and a little while later, my cousin put on her towel and started to go up to my grandparent's house to get an ice cream sandwich and my grandpa said, "Hannah, take off your towel-you need the sun." when he and I knew perfectly well that the sky had clouded up. Is this normal, and what should I do?