No you don't need the mothers DNA. If he wants the child matched with him he needs to provide swabs and so does the child. I don't think he needs the mothers permission if he does it when he has the child in his care. On a visit etc. I might be wrong - but I don't think it requires both parents consent.
Just be aware that if you do this, it will not be recognised by a Court as it has been done without the other parent's consent. It may also have other ramifications.
There are appropriate legal procedures to follow if your brother really doesn't believe he is the father, and he needs to obtain independent legal advice pertaining to his own situation.
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