Dreams of shifting identities -- and there are two instances of them in your dream -- often are confusing. If you look closely, however, you can almost always discover the link that explains the metamorphoses between characters.
The dominant theme of the first part of your dream is the pride you felt on your wedding date vis-à-vis the "ex" who lost the battle for your husband's hand. This ex, significantly, is not directly represented. Though you know she is the "ex", in the dream she appears as a woman who dated one of your husband's friends -- whom you liked.
This character's identity shifts again when, after she jumps to her death, you discover that she is your brother. It is significant again that, in the dream, you do not immediately rush to the aid of your brother -- a detail you find abhorrent in waking life. Instead, you are more concerned that you might get in trouble. In your own words, you "fear someone would find out I caused this."
Given the context of this dream, is it possible that the woman who jumps to her "death" secretly represents your brother's wife? This would explain her "disguised identity," and it would explain the associative link for her subsequent metamorphosis into your brother. This interpretation also explains the marriage setting of the dream, wherein you successfully defeat a rival for your husband's (brother's) attention.
Death in dreams is a common symbol for change, and for making something or someone "go away." Accordingly, your dream should not be interpreted to mean that you actually wish your sister-in-law would "drop dead." It does suggest, however, that occasionally you wish she would just "go away." Similarly, your brother's "death" should not be interpreted literally. On the contrary; it symbolizes the change that has occurred in your relationship with him.
It is clear that you feel displaced in your relationship with your brother by his wife. When siblings are close, it is natural to feel territorial, and even jealous, of "competition" for the primary relationship. Your dream, however, shows that you have mixed feelings about these competitive emotions. On the one hand, you would like to express them (the "in-your-face" portion of the dream); on the other, you feel guilty about them and are "afraid of being caught."
At its conclusion, the dream shows that you feel responsible, at least in part, for causing the change in the relationship that has occurred between you and your brother. Is it possible your "territorial instincts" have gotten the best of you? Your dream suggests it is time to re-examine your role in this competitive dynamic, and make room for the newest member of your family.