Welcome to Susan's Place. A few bits of information. Being a member of the LBGT family can run in the family. As you have one member, it's more likely to have a second member. Next CIS don't think about or question their gender identity. The fact that you have been uncomfortable with your gender identity strongly indicates you are transgender.
You haven't posted your age but from the way you think, I can tell your fairly young. At some point in your life, you need to learn you can't be everything to everybody else. You need to find your own happiness and that means your younger sister may have to make a compromise. You might still walk her down the isle but the image might not match what she currently sees in her mind.
I have two links you might find useful. The first is
our WIKI where you can learn more about being transgender. The second is
"the transition channel" where you can explore your feelings.