I think you should actually talk to your girlfriend first, if anyone. Speaking as a girl, I would MUCH rather hear from my boyfriend he is having feelings for another anonymous girl than my sister. If you don't tell her all the gruesome details, her relationship won't be strained with her sister. At the same time, whatever female friends you have she'll become immediately jealous of.
It sounds like you'd be content, and maybe even HAPPY (!) marrying your girlfriend. Right now, though, you're finding someone else more attractive. I have news for you. Even in the most committed married couples, BOTH (!) partners will see people on the street who catch their eye, and they'll wonder, "What would life be like if things were different?" Of course, that doesn't mean their life isn't bad.
I think butterflies are a bad thing. I don't think they show the truth, they just show you're getting a random feeling of attraction for someone, to where it makes you a little uncomfortable. I get those when I'm nervous around people, sometimes attractive people. I get them when I speak in front of people, from the exhilaration of speaking in front of people, which I really like. Would I like to live my life public speaking? Fuck no. I don't get them with my boyfriend. I feel comfortable and safe with him. Kids in high school get butterflies over random crushes all the time. I even just asked my boyfriend about this, and he says it means nervousness or just a little crush.
While I feel that I love my boyfriend, there are ALWAYS times when there are people I'm talking to and I get that spark... But I would never sacrifice a relationship I know is working and makes me happy for someone I don't know as well or...well, you get the idea.
Nothing I can say can convince you, I know, so I'll just end with this last part. My sister and I are absolute best friends, and we're on similar maturity levels. I'm the little one. If my sister's boyfriend told me in the most mature way possible he had feelings for me, I would freak out. Even if he has been mature and good about it. My sister will ALWAYS come first, and she'll deserve to know. She'd react much worse to hearing it from ME, and any girl will feel betrayed. A girl who has to hear from someone else you've got other feelings will feel like you don't trust or love her enough to be honest. I'd never take any "leftovers" from my sister, and the only girls who would would generally take whatever slime they can get.
Do NOT tell the sister first, okay?
(also if you tell the sister, there are two drastic outcomes: a)she'll like you and "steal" you, and ruin the relationship with her sister and as people have mentioned you won't be welcome in the family and b)she'll tell her sister and her sister won't trust her because she'll feel like her sister is more attractive and better than her. Trust me, that's how all girls work.)