I have been in a relationship for almost four years, and we are serious about marriage. My beau and I love each other a lot, but his sister, who is younger than both of us by two years, detests me. She finds faults in all that I do, in my behavior as well as everything else. I also strongly feel that she is jealous of me. She is aggressive, short-tempered, and stubborn. I am a very emotional person, and I am getting disturbed by what she says. Please help me.
In every marriage, a lot of people come together and get entangled in each other's lives—the two people exchanging vows, family members both close and distant, friends, animals, and even the inner figures that play a part in the lives of us all (your inner mother, weakling, hero, or adventurer, for example). So your starting point should be to be realistic about the future: If you get married, you'll be connected to your fiancé's family, including his sister, for the rest of your life.
The next step should be to find ways to deal with your emotions. Calling yourself "emotional" doesn't help. We are all emotional. But for you, this self-description might mean that you get caught in other people's criticisms. If that's the case, you have a more specific job at hand—learning to live with the fact that not everybody will like you in life.
One way to deal with another's dislike of you is to learn how to be loyal to yourself—and especially to whatever qualities are being challenged. Find the strength to be firm in who you are no matter what someone else might say. Criticism is always a two-way street: one person complains, and then the other person caves in under the attack. Your job is not to cave in but to be proud of who you are.
One further response might be to make it clear to your fiancé what a big problem his sister is. He also has a role to play in improving the climate between you and his family. Your letter to me suggests that you may not be the kind of person who speaks her feelings loud and clear. Now would be a good time to work on that.
I work by the principle, “Go with the symptom.” If you feel too emotional, the solution is not to try to be cool and contained. Instead, you should respect your feelings: speak for them, make them known, use them as guides to let you know what you need.
In other words, it's time to flex your psychological muscles. Don't be not defensive, but be firm and clear. Best of luck in your relationship.