If your brother's domain is registered at some registrar other than DreamHost, then you can easily arrange the transfer from within the DreamHost panel at Domains -> Reg. Transfer.
However, if your brother's domain is registered under another DreamHost account, then I believe you will have to ask DreamHost support to do the transfer for you.
Yes, you can host unlimited domains under your DreamHost account.
Once the domain transfer has completed, you just need to add the domain to the DreamHost hosting system via the panel at Domains -> Manage Domains -> Add New Domain / Sub-Domain.
If you wish to keep this domain separate from your existing domain, you can create a new user for your brother and specify this user when adding the domain to the DreamHost hosting system. This way, when your brother logs in via FTP, using this new user, he will only 'see' the contents of his own home-directory, including the web directory for his domain(s).
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