Given the attempts at harassment and assault by the husband's brother, and repeated threats that have been disregarded, along with the inability to stop him except by informing the extremely jealous husband, which could lead to major problems, what should I do?

It is not permissible to remain silent about the wrongdoing of the husband's brother. All actions that could lead to forbidden acts, such as being alone with him (khalwa) or mixing with him (ikhtilat), must be avoided, and he should be prevented from entering. The husband's help should be sought in this matter without initially informing him of what his brother is doing, because the husband is responsible for protecting and safeguarding his wife. The Sharia has strongly warned against leniency in a woman's interaction with the husband's non-mahram relatives, and "al-hamwu al-mawt" (the in-law is death) means that being alone with a husband's relative is more dangerous than being alone with someone else. If these methods do not work, then whoever can deter the husband's brother should be informed, such as your husband's father. If there is no way to deter him except by informing the husband, then he must be informed so that he can prevent this wrongdoing.