Probably because you are a lonley person who has no real social life. You should learn that there are more appropriate females for both sexual fantasies and for real life social experience. What you're doing isn't particularly healthy.
I totally agree with Redbeard. You didn't mention what age you were. In my teens I was almost 7 years older than my brother and I would notice him every so often taking quick peeks at me while I was undressing. (A good door slam solved the problem! LOL) It's fairly normal when younger because of curiosity, but if you are 16 and older then you have a problem!
I disagree with the above thoughts on your question. There is no wrong or right here. You feel what you feel, period. However, the cause has to do with your sexual energies combining with your sisters, they are similar yet complimenting enough for you to bring up this attraction, and it's taboo so all the more off limits. This has excited you. No bigee. If you both got together these energies would most likely dissipate and your attraction would move elsewhere. But since society frowns on such behaviour with severe judgement, it would be better for you to direct your sexual energies towards other interests. Steer your attraction to other woman and men who you also find attractive. As you express how you feel towards them, they will change and you will learn a lot about your self and what you really want and like. All the best to you.