Is it normal that i have a strong sexual attraction to my gf's sister?

I've been with my girlfriend for almost 4 years. She has a younger sister, by a couple years, and we all spend a lot of time together. I find the sister more attractive than my GF and constantly daydream and even dream about her. At times I feel I have more in common with the sister than my GF. I almost feel a mutual attraction. Could just be me, but I sure do catch her staring at me a lot. I love my GF, but I can't stop thinking about the sister! I feel horrible for saying this, but I almost feel that I wish I had meet the sister first. Don't know what to do. I could just pursue the sister, but that would completely f$%K s#!t up! I appreciate any thoughts or ideas or even anyone in the same position. Thank You!