OP, your feelilings are understandable, you didn't grow up together and have a normal sibling relationship, so don't feel too bad about this and please play no mind to the people who are saying it's gross etc.
However you really need to rationalize this situation and see that your sister has a boyfriend whom she presumably loves so probably does not feel the same way about you, and even if she didn't have a boyfriend, it's highly unlikely that she would feel the same way. Telling her how you feel would most probably destroy your relationship with her forever, I don't see how i could be anything other than incredibly awkward. And yes, your mom would probably have a heart attack.... the truth is, revealing your feelings won't result in anything other than horrible awkwardness, so you will need to keep these feelings to yourself. trust me, you will survive it and you will eventually fall out of love with your sister, I know this seems impossible right now but these feelings will fade. And again, don't beat yourself up about it too much. You will find another girl eventually.