My brother installed kali Linux on my computer. I believe he forgot his user name. How do I get around it to use my laptop

Log in as root and look at /home directory, there will be a folder with same name as your brother's username.

If your brother can't remember his username and password, I suspect he doesn't remember the root password either.

You can boot to a Live CD of something, mount the laptop hard drive, then follow Midou's suggestion. Alternatively, you could take a look at System Rescue CD. I've not used it, but it's designed for just such situations.

If this laptop is dual-boot with something else, boot into the other OS and blow away the Kali partition. Your brother shouldn't be putting Kali on your laptop, anyway. Like fire, Kali is not a toy and not to be toyed with.