You are acting as though you and your sister-in-law are in a competition. In reality, there is no competition. You are not rivals. You are dating her brother; there is no competition.
You seem to believe that she is superior to you. It is important to recognize that beauty is in the eye of the beholder. One person may find her more attractive than you but the next may find you more attractive. Attraction is dependent upon what one likes and dislikes; it varies greatly from individual to individual. There is no one standard of beauty.
I had a wonderful college professor who used to conduct an exercise in class to prove that there is no one standard of beauty. He would present a scenario to the class in which students were stranded on an island, alone and never to be saved. Each student was then asked to choose a celebrity they wanted to live with them on the island. Interestingly, each time he conducted this exercise, there were a wide variety of choices. Rarely did students choose the same celebrity. He conducted the exercise for the purpose of demonstrating how unique each person’s attraction is to another. If there were one standard of beauty, there would have been no variation in the students’ choice.
You also make the point in your letter that you have a high opinion of yourself (“I am a good-looking and intelligent young woman”…) but if you truly felt this way then you would not be jealous of your sister-in-law. Jealousy is an issue of low self-confidence and feelings of inferiority. It occurs when people do not have high self-esteem.
It is important to deal with this jealousy problem, otherwise it will continue to degrade your life. One way to handle this situation is to work on improving your self-confidence. I would advise you to read books about self-esteem. I would also suggest paying less attention to other people (especially your sister-in-law). I understand this can be difficult. With practice you may learn how to accomplish this task. Another idea is to make a list of accomplishments, to serve as a daily reminder of your positive qualities and successes.
You were able to stop obsessing about your sister-in-law once but as you noted, you’re doing it again. If you continue to struggle with these issues, psychotherapy could help. Click on the find help tab at the top of this page to search for a therapist. Please take care.