First, you may not be the one entitled to receive the proceeds.
Federal and state law may provide that your brother's surviving wife is entitled to some or all of the 401(k) unless she waived her rights to it. In your place, I would not sign the forms from the plan administrator. Instead, I would inform them of his marriage and inquire as to whether they have a spousal waiver on file.
Second, if you do receive the proceeds and your brother's tax debts go unpaid, you may be held liable under a theory of "fiduciary liability."
A local attorney versed in retirement benefits and taxes can help sort this out for you.