Hello jatin196,
Welcome to Apple Support Communities. It sounds like you're having trouble with your brother being able to unlock your iPhone with his face. We are happy to share information that may help.
"The probability that a random person in the population could look at your iPhone or iPad Pro and unlock it using Face ID is approximately 1 in 1,000,000 with a single enrolled appearance. As an additional protection, Face ID allows only five unsuccessful match attempts before a passcode is required. The statistical probability is different for twins and siblings that look like you and among children under the age of 13, because their distinct facial features may not have fully developed. If you're concerned about this, we recommend using a passcode to authenticate."
Check out the "Security safeguards" section of this article for more information: About Face ID advanced technology.
A step to take toward resolving this is to reset Face ID and set it up again: Go to Settings > Face ID & Passcode and tap Reset Face ID. Then tap Set up Face ID to set it up again.
Take care.